Revert "Frame: Fix RendererHasFocus"

This reverts commit ff918df88991710d58d983aa2e5dbcf03f2064ab.

This changed it from "RendererHasFocus" to "UIHasFocus", which is
wrong. Specifically, it broke for non-Render to Main Window cases where
the renderer window isn't managed by wx. It also broke the pending
exclusive fullscreen support, which checks this function to determine if
the renderer is on top so it can full-screen it.

We'll add a new hook, "UIHasFocus", in the next commit.
Host: Add a new "UIHasFocus" hook to determine if the UI has focus

We can't use RendererHasFocus for this purpose because of some issues
with exclusive fullscreen, and the new RendererHasFocus implementation
didn't work for non-Render to Main Window cases, since the renderer
window wasn't managed by wx.
Merge pull request #633 from magcius/wip/input-focus-fix

Fix input focus issues