Use an edit control in the keyboard dialog

This takes care of various bugs/sillies that have been quite long standing (e.g. weirdness with long labels input into the keyboard dialog, masses of duplicate code etc.)

It also means we can drop the silly "remote sends keyboard presses" setting that is confusing to users.

Note that this does not address the numeric dialog, which is a different issue, but could potentially be remedied in a similar manner (thus we need to leave some code around in CGUILabelControl).

@koying: this should mean #4874 shouldn't be needed.
@ronie, @BigNoid, @Black09: needs confluence change, but should be fully backward compatible anyway.

Obviously this needs good testing, particularly on systems without a keyboard.