Be consistent about touch point id type: use qint32

Summary:
There is no point in using quint32 and casting back and forth in numerous places.
Fix a bunch of compiler warnings that we implicitly cast between signed and unsigned.
This makes things consistent with what we get from libinput.

Reviewers: #kwin, romangg

Reviewed By: #kwin, romangg

Subscribers: zzag, romangg, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D23086