Fixed rendering bug

If multiple intersecting rectangles are to be redrawn on
a single page, the rendering engine used to redraw only
the intersection of the rectangles (not enough) instead
of the union (enough). This problem became appearent after
undoing / redoing a "delete stroke" operation on multiple
letters

Signed-off-by: Wilson Brenna <wbrenna@uwaterloo.ca>
Added tool item for page deletion

Signed-off-by: Wilson Brenna <wbrenna@uwaterloo.ca>
This hopefully fixes xournalpp/xournalpp#41

Signed-off-by: Wilson Brenna <wbrenna@uwaterloo.ca>
Scroll to begin/end using Home/End keys

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