Fix Qt::MouseButton Documentation for Qt::ExtraButton24,

Fix Qt::MouseButton Documentation for Qt::ExtraButton24,

Documentation for the 'more mouse buttons' feature has a Typo on
Qt::ExtraButton24 (is shown as "Qt::ExtraButton44", no value.)
Also, the order of BackButton/Xbutton1/ExtraButton1 causes the
'BackButton' value to be a forward reference to XButton1, which
makes it awkward and hard to understand. Same with ForwardButton/
XButton2/ExtraButton2.

Task-number: QTBUG-25529

Change-Id: I04b76d91d7a2155c1917a809a51700059320f510
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>
Reviewed-by: Casper van Donderen <casper.vandonderen@nokia.com>