Changes made in property dialogs now apply immediately

This affects the custom property editing as well as the editing of any
built-in properties for classes like ObjectGroup, ImageLayer, Map and
MapObject. To make this possible, some new undo-related classes were
introduced and some new notification signals.

This change is a preparation for allowing the property dialogs to be
replaced by a docked solution.
WIP: Added PropertiesDock

Displays properties of selected objects and tiles at the moment.

TODO: Display properties of layers/map/tileset.
TODO: Display other stuff like size, height, color, etc.
(amend) Select current layer when it changes or is pressed
(amend?) Unbreak undo, somewhat

The silly thing is that undo in the text field will cause a new undo
command to be created -> not integrating well with the system.
Added the Qt Property Browser solution

A planned replacement for the current primitive property editing table.
Make the .bat file always check out with Windows newlines
WIP: Property Browser
WIP
WIP