VE-1655: Preview functionality

Adds preview functionality to the single media tool:

**ve.ui.Surface.js**
* Add a getter for the globalOverlay

**ve.ui.WikiaMapInsertDialog.js**
* Listen to new event

**ve.ui.WikiaMediaInsertDialog.js**
* Use globalOverlay getter instead of DOM traversal
* Listen to new events
* Adjust onSearchCheck to use passed item instead of itemData

**ve.ui.WikiaSingleMediaDialog.js**
* Integrate ve.ui.WikiaMediaPreviewWidget
* Handle events
* Correct UI for checked and unchecked results

**ve.ui.WikiaMediaOptionWidget.js**
* Added new events for mousedown on metadata and label. In the single media dialog, clicking these element should prevent the select widget from emitting 'select'. To do this, the DOM event is passed through EventEmitter up to the dialog, where the dialog can choose to stopPropagation. This is so a click on the label in the old media dialog does not prevent the 'select' event from firing.

**ve.ui.WikiaMediaResultsWidget.js**
* Event handling