Frame: Add "Render To Main Window" Exclusive Fullscreen support.

This is a second attempt rewritten from scratch, the previous attempt was PR #703. The basic idea behind this branch is to reparent the render frame into the main frame as opposed to rendering to a panel. And remove the parent when going to fullscreen, thereby no longer requiring a separate fullscreen code path for "Render To Main Window".

As a side effect this PR also fixes the hotkeys when Render To Main Window is enabled.

I have not yet tested builds on Linux or Mac, since it relies on quirky wxWidgets behaviour it should be tested on those platforms before merging this PR.