QML: fix clang warnings about unused return values

For example calling toRect() and throwing it away caused this warning
error: ignoring return value of function declared with
warn_unused_result attribute [-Werror,-Wunused-result]

Change-Id: I849088d052d21402a5f50cfe9d1305c3e1d91244
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Qt Quick: use categorized logging for touch, mouse, focus, and dirty

There was no need to log debug info as warnings; and QTouchEvent has
had a streaming operator for some time, which shows more useful info.
Categorized logging makes all of this debug info available without
rebuilding Qt first and without adding new env variables.

Change-Id: If2f5bab3f9d90c8b72b295c075e7184662d3ead3
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>