xcb: qxcbdrag.cpp cleanups

- moved finding of XdndAware target logic in its own function to
  reduce size of QXcbDrag::move().

- switched to use categorized logging with more consistent logging messages

- added more comments to avoid constatly looking at the
  specification for the meanings of Xdnd* actions and who
  sends/receives the action.

- removed dead code (findXdndAwareParent), which should have been
  removed in 269fdbdd2bedda5f5eacb751224d3a3fc3eed5bc when reimplementing
  this logic in XCB.

- removed needless reseting of state variables in various places as
  this is handled in QXcbDrag::init() on DnD start.

- renamed variable in QXcbDrag::dndEnable(): xdnd_widget -> window

- and other minor cleanups

Change-Id: Ib667f80ceb4c07b7409a90c041044c98665877f3
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

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