I simplified a lot of API calls by pulling panel memory management
inside the library. All API calls which previously required a panel
as parameter are now handling their panel themself.
Previously if you used movable or scalable groups you had to add
the group border width to the panel to access the updated position
or size. Nuklear now takes control of the process and the new
position/size can be read after `nk_group_end` from `nk_panel`
directly.
All platform and renderbackend independent examples are now optional
demos for all platforms in `demo/`. Moving some demos from `example/`
provides an easier way to test GUI concepts for users and simplifies
maintenance.
