Fetch StandardKey shortcuts from QPlatformTheme

This allows platforms to dynamically alter their keyboard
shortcuts, for example if they are user-configurable on that
platform. Current behavior remains the same.

QEvent previously used the hardcoded values in QKeySequencePrivate so
this was modified to use QKeySequence::keyBindings().

In order to keep the speed of QEvent's former binary search, we moved
this code to QPlatformTheme::keyBindings(), making it faster for all
keyBinding lookups.

As we now need to search by StandardKey instead of by shortcut the list
is reordered and a test is changed to reflect that.

Change-Id: Iefb402fbbe8768be2208ce036f3c2deed72dcc6c
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>