Modifying the size of a section

Added the ability to resize sections. By default, tiles are added to the right (if you extend the width) and top (if you extend the height). Similarly, shrinking width or height will truncate tiles from this direction. I've added check boxes so that this behavior can be inverted such that the left or bottom is extended/cropped instead.

The resizing is done through the section properties form, which can be accessed through the Section menu.