Extract markdown insertion logic out of XGSTextViewController.

Insertion logic is now in XGSMarkdownInsertionController.
Clarify API for the keyboard shortcuts

 Creates a protocol XGSCircularCellItem, the cell knows how to handle it.
 Create XGSShortcut class to represent a string shortcut with a
 custom icon representation in the accessory view.
 3 classes implements XGSCircularCellItem: MarkdownTag, Shortcut and NSString.
 Any of those can be used in the `shortcuts` property of the input accessory view.