Added an utility class for connecting signals

It monitors a GProperty of a GObject that should be of
a GObject type and automatically disconnects and
reconnects signal handlers when that GProperty changes
MouseHandlerFactory now has a CATEGORY constant
Added the magnifier component
Added a scrolling mouse handler that changes the magnifying glass
Added a "new-view" signal to the application class
Fixed copying images not working
Adding copy_to_clipboard to ImageSource documentation
Added an outline to the magnifier
Added option to magnify the background
Added "crossing" events to mouse handlers
Magnifier glass now hides when point leaves the view
Modified how the key filter for mouse handlers work
Removing "hovering" as a mouse handler event, added "moving"
The magnifier now changes size based on its magnification

It also hides when its magnification is less or equal 1
The magnifier can now be shaped as a square
Re-added magnifying glass radius change in the scrolling handler
Fixed the mouse handler filter to work as intended
Added settings widgets' replacements for mouse handler dialogs
Fixed bug in PointScale widget

The bug made it not work with ranges that go into negative values
Also made its "square" attribute into a property
Added PointProperty utility