Add containsPress property to MouseArea

It is very common to use pressed and containsMouse properties
together to highlight a pressed item, e.g.

    property bool highlighted: pressed && containsMouse

The containsPress property allows simplification and optimization of
user code.

[ChangeLog][QtQuick] Add containsPress property to MouseArea

Task-number: QTBUG-40130
Change-Id: Ie286d431154eb37a99e57e4cf881d68d7cbbe31d
Reviewed-by: Martin Jones <martin.jones@jollamobile.com>