Commercial games assume that this value is 1 but they never set it. On
the other hand nouveau manually sets this register. On
ConfigureFramebuffers we were asserting for what we are actually
implementing (according to envytools).

With shader caches on the horizon, one requirement is to provide visible
feedback for the progress. The shader cache reportedly takes several
minutes to load for large caches that were invalidated, and as such we
should provide a loading screen with progress.

Adds a loading screen widget that will be shown until the first frame of
the game is swapped. This was chosen in case shader caches are not being
used, several games still take more than a few seconds to launch and
could benefit from a loading screen.

This isn't used at all, so we can just get rid of it.

Allows removing the inclusion of the main input common header from the
UI config header.

'this' isn't actually used within the lambda, since what we need
from the class is already assigned within the capture section of
the lambda.

It's a common UI pattern to use an ellipsis to indicate an ongoing
action, rather than just specifying the word by itself.

Gets rid of a few indirect inclusions.

This is a function that definitely doesn't always have a non-modifying
behavior across all implementations, so this should be made non-const.

This gets rid of the need to mark data members as mutable to work around
the fact mutating data members needs to occur.

Avoids the need to potentially reallocate the contained callbacks.

These currently aren't used by anything other than the QtWebBrowser
class itself, and can be made private.