* implemented BControllable::StartControlPanel(). The behaviour is supposed to be the one documented in the BeBook, meaning that it first checks if the BControllable node has been instantiated over an BMediaAddOn. If so, we investigate where the addon is located and try to launch it. Thus media_addons which have a control panel need to be compiled as an Application, not as an Addon. See r24664 as an example.
* inside BControllable::HandleMessage() call StartControlPanel for the message.
* MediaNodes having such a ControlPanel are rarely used as parameterwebs should provide you everything. But for instance the vst MediaAddOn uses it and some others do as well.
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@24665 a95241bf-73f2-0310-859d-f6bbb57e9c96