FrameTools: Add a wxPanel as parent of the RenderFrame on Linux.

Needed for hotkeys to function correctly on Linux.

I have no idea why it's necessary, it doesn't look like it's caused by either wxTAB_TRAVERSAL or CFrame::RendererHasFocus().

Unfortunately I can't confirm what the cause is, because gdb is generating segfaults on my Ubuntu 14.04 VM when debugging dolphin. Maybe another linux dev could take a quick look at it?