Add 'AllButtons' into Qt::MouseButton enum, for use by QML

This allows the QML programmer to set 'acceptedButtons', for a
MouseArea, to accept all buttons with a single value. In
comparison with OR'ing a long list of Qt::MouseButton values,
this is shorter, easier, more clear, and less error-prone.

Task-number: QTBUG-24106
Change-Id: I0259969223c0b44b6ce8ae84aed37d20cb77999b
Reviewed-by: Alan Alpert <alan.alpert@nokia.com>
Reviewed-by: Martin Jones <martin.jones@nokia.com>

src/corelib/global/qnamespace.h

@@ -184,6 +184,7 @@ public:
         ExtraButton22    = 0x01000000,
         ExtraButton23    = 0x02000000,
         ExtraButton24    = 0x04000000,
+        AllButtons       = 0x07ffffff,
         MaxMouseButton   = ExtraButton24,
         // 4 high-order bits remain available for future use (0x08000000 through 0x40000000).
         MouseButtonMask  = 0xffffffff