Minor change, rename MovingPixelState::m_currentEditor -> m_editor
Fix selection tool behavior when Shift key is used

There was a conflict between Shift key as a modifier to Add selection and
to convert the selection box with square-aspect. Now tools::Controllers
can process key down/up events, so the square aspect is used only if
TwoPointsController receives a Shift key event (in the middle of the
tool loop).
Deselect with a simple click
Add special mode (xor black/white) for selection tool (fix #301)

This change includes a blend mode for the document extra cel, but it's not
used to fix the issue.