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.
Added a Properties dockwidget

Displays properties of selected objects, tiles and layers at the moment.
Does not display other stuff like size, height, color, etc. yet. My plan
is to do that using the Qt Property Browser solution.
Added the Qt Property Browser solution

A planned replacement for the current primitive property editing table.

Based on qt-solutions revision 8926435d3f2b94ebfadc1bec5fd78cf0dfa5c2c8.
Introduced a PropertyBrowser for in the Properties dockwidget

The PropertyBrowser class extends QtTreePropertyBrowser and allows
editing of custom properties as well as built-in properties like layer
names, object size/position, etc.

There is currently no way yet to add/remove custom properties.

The plan is to allow the Properties dockwidget to replace the Properties
dialog.
Set the tree widget as focus proxy of the tree property browser

Makes the focus go to the tree widget when set on the property browser,
since it's not very helpful for it to stay on the property browser
widget.
Added actions for adding/removing a property to Properties dock

Also made Delete and Backspace work for deleting the selected property.
Unset the current object when it gets removed