Summary:
Pass screen index to TabletHandler::onScreenRotated, and check if the
tablet is actually mapped to the screen that has been rotated.
If no screen is specified, then auto-detect rotation using QScreen.

Remove all xrandr mentions since it's not used anywhere anymore.
Clean up X11Info class.

Test Plan:
See if there aren't any regressions
Multi-screen rotation handling seems to be still broken
because QScreen doesn't seem to fire rotation events
for non-primary screens (probably https://bugreports.qt.io/browse/QTBUG-65598)

Reviewers: xuetianweng, fvogt

Subscribers: ngraham

Differential Revision: https://phabricator.kde.org/D9719