Add GitHub Issue Templates

- bug_report.md: template for bug reports
- feature_request.md: template for feature requests
- fltk-build-or-usage-question.md: not really a template [1]

[1] Explain to users that this is the wrong place to ask user
    questions and refer them to fltk.general

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,58 @@
+---
+name: Bug report
+about: Create a report to help us improve FLTK
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**To Reproduce**
+Please attach a self contained example that allows us to reproduce the problem.
+Such an example typically consists of a single source file (e.g. `demo.cxx`) and
+build instructions (e.g. `fltk-config --compile demo.cxx`). Alternatively, try
+to reproduce the bug with one of the included test programs in the `test` or
+`examples` folders. Add a description like this:
+
+Steps to reproduce the behavior:
+1. Go to '...'
+2. Click on '....'
+3. Scroll down to '....'
+4. See error: ...
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**Screenshots**
+If applicable, add screenshots to help explain your problem.
+
+**FLTK Version**
+Please complete the following information and delete non-applicable lines:
+ - Version: [e.g. 1.4.0]
+ - Installed system version from: [e.g. Debian 11.4 Bullseye]
+ - Downloaded and self-built from: [e.g. Git, Snapshot and version, or tarball/zip]
+ - If from Git, branch: [e.g. master, branch-1.3]
+ - If from Git, commit: [e.g. 7d58e2385452]
+
+**FLTK Configure / Build Options**
+Please add the complete configure and build command lines if you built FLTK yourself.
+Please include all configure or CMake options and add all CMake options you changed
+by using CMake GUI tools (if any).
+ - ./configure --enable-cairo ...
+ - cmake -G"..." -DCMAKE_BUILD_TYPE=Debug ...
+
+**Operating System / Platform:**
+Please be as precise as possible, e.g. "Linux: Ubuntu 20.04"
+ - OS: [e.g. Linux, macOS, Windows]
+ - OS Version: [e.g. Ubuntu 20.04, macOS 12.6, Windows 11]
+ - Processor if relevant [e.g. on macOS: M1, Intel]
+
+**Linux/Unix Runtime, if applicable:**
+ - Wayland
+ - X11
+
+**Additional context**
+Add any other context about the problem here.

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for the FLTK project
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**Is your feature request related to a problem? Please describe.**
+A clear and concise description of what the problem is.
+Please make sure that the desired feature is not available in a later
+FLTK version (e.g. 1.3.8 or 1.4.0).
+
+**I am using FLTK**
+ - Version [e.g. 1.4.0]
+
+**Describe the solution you'd like**
+A clear and concise description of what you want to happen.
+
+**Describe alternatives you've considered**
+A clear and concise description of any alternative solutions or features you've considered.
+
+**Additional context**
+Add any other context or screenshots about the feature request here.

.github/ISSUE_TEMPLATE/fltk-build-or-usage-question.md

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+---
+name: FLTK build or usage question
+about: Please use our FLTK discussions group (forum) fltk.general for your question!
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**Please do not use GitHub Issues for FLTK user questions**
+
+This is the wrong place to ask questions about building FLTK or running programs with FLTK (user support). FLTK "issues" are reserved for bug reports and requests for enhancements, for further information please visit https://www.fltk.org/bugs.php.
+
+Please ask user questions in our user group [fltk.general](https://groups.google.com/forum/#!forum/fltkgeneral). Your question will have a wider audience and you will likely get more and quicker replies.
+
+Please include the system/platform (e.g. Windows, macOS, Linux/X11 or Linux/Wayland), the build system (e.g. Visual Studio, CMake, Ninja), compiler (gcc, clang, ...), compiler version and build error messages in text form (copy and paste preferred). Thank you.
+
+Note that FLTK user questions here (in GitHub Issues) **must** be closed immediately by a maintainer with label `no user support`.