Rename this.$ to this.$element, and this.$$ to this.$

Objectives:
* Rename this.$ to this.$element
* Rename this.$$ to this.$
* Get rid of the need to use this.frame.$$
* Rename OO.ui.Element.get$$ to OO.ui.Element.getJQuery

Changes: (using Sublime Text regex patterns)
* Replace "get$$" with "getJQuery"
* Replace "\.(\$)([^\$a-zA-Z])" with ".$element$2"
* Replace "\.(\$\$)" with ".$"
* Replace "'$$'" with "'$'"
* Set this.$ to null in constructor of OO.ui.Window
* Set this.$ to this.frame.$ in initialize method of OO.ui.Window
* Replace "\.(frame.\$)([^\$a-zA-Z])" with ".\$$2"

Bonus:
* Use this.$() in a bunch of places where $() was erroneously used

Change-Id: If3d870124ab8d10f8223532cda95c2b2b075db94
Merge "Rename this.$ to this.$element, and this.$$ to this.$"