Reimplemented scrollbar hiding

As requested in xournalpp/xournalpp#98 this adds the
possibility to hide scrollbars which might be useful
for tablet users. I noticed however, that this
has quite some undesired side effects, namely scrolling
does not seem to work properly anymore. We should
investigate this further before pushing anything into
mainline...

Signed-off-by: Wilson Brenna <wbrenna@uwaterloo.ca>
Improved scrollbar hiding

Scrolling works correctly with arrow keys while the scrollbars
are hidden; however, the hidden scrollbars don't respond to
scroll events as are generated by mouse wheels.

Signed-off-by: Wilson Brenna <wbrenna@uwaterloo.ca>
Fixed conflict in glade from scrollbar patch