xcb: QTabletEvents and proximity events correctly identify the tool

The tool is enum TabletDevice reported in QTabletEvent::device().
Each type of tool for each type of tablet has an ID which needs to
be mapped to this enum.

The meaning of the AbsWheel valuator depends on the tool type:
an airbrush sends tangentialPressure in the range -1..+1,
whereas an Art Pen (RotationStylus) sends rotation in degrees.

Because the evdev driver sends valuator changes individually, we
have to store the current value in each TabletData::ValuatorClassInfo
to ensure that each QTabletEvent has complete state information.

[ChangeLog][Platform Specific Changes][X11 / XCB] QTabletEvents
and proximity events correctly identify the tool in use; the
meaning of certain valuators depends on the tool; and the valuators'
current values are not lost between events

Task-number: QTBUG-39666
Task-number: QTBUG-39573
Task-number: QTBUG-39570
Change-Id: I6be242ee57e2ad0848060e04f20c0dd08954a734
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Doc: edited qtxml.qdocconf

Added dependency for Qt Widgets

Change-Id: I36fbec0738f5671a07b9dd18fc4487573c560cd2
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>