Renamed variables for improved readability.
Unit-tested drawing methods in PaintingWindow.
FIX: corrected PaintingWindow::drawText().
FIX: corrected sending of Qt signals.
MAINT: Refactored unit testing for PaintingWindow.
FIX: added testing stub for PaintingWindow events.
Merge remote-tracking branch 'origin' into test-painting-window
FIX: fixed unit test for mouse move events.
MAINT: reformatted lines of codes.
MAINT: code cosmetics.
Added more unit tests for mouse and keyboard events.

N.B.: this will likely fail in Travis CI.
FIX: fixed strange platform-specifi behavior of mouse move event.
Merge remote-tracking branch 'origin' into test-painting-window
MAINT: reformatted lines to 80 characters.
MAINT: removed commented line.
FIX: increased waiting time for the signal reception.
MAINT: fixed indentation.
Worked around a platform-specific widget position bug.

This seems to be the case for Mac OS X.
MAINT: used `QSignalSpy::wait()` instead of `QTest::qWait()`.
MAINT: reformatted lines of code.