Selections: Add Selection class inside new util module

Moves ext.cx.util.js to util module

Adds Selection class with the following methods

* get - captures current selection
* save -  saves a selection to key-value store
* restore-  restores a saved selection
* pasteHtml - pastes html at current or restored selection

Update Resources.php to load the Selection class for:
* ext.cx.translation
* ext.cx.tools

Selection class instance available as:

mw.cx.selection

Change-Id: Ic7b7a0d7d2869f256005ee41b916aac39a977b2e
Merge "Selections: Add Selection class inside new util module"