Fix OpenGL video backend when using "render to main" on Windows.
Don't create a Panel for a seperate render window.

Instead of setting RenderParent to null and raising complexity RenderFrame can be its own parent.
Remove CPanel.

Let the RenderFrame handle those messages.
Remove wxTAB_TRAVERSAL style from RenderFrame.
Fix RendererHasFocus().
Remove g_pWindowHandle.

Now that the EmuWindow is gone we don't need the global anymore. The Host should provide a window handle for the renderer.
Update documentation.