Restore Wintab support

Summary:
Basically, merging Qt's qwindowstabletsupport.h into Krita.

I have only tested this with a modified version of Qt. I comment out
the following line in QWindowsContext::QWindowsContext():

// d->m_tabletSupport.reset(QWindowsTabletSupport::create());

this is found at line 347 of qwindowcontext.cpp in my local copy.

The reasoning behind that change is as follows. Qt creates a hidden
dummy window to receive tablet events, and I copy that methodology.
I do not want to worry about conflicting tablet contexts, so I
disabled Qt's extra window, leaving only our own.

Reviewers: dkazakov, rempt

Maniphest Tasks: T958

Differential Revision: https://phabricator.kde.org/D545