Link improvements PR

Several improvements for link editing in the new visual editor, as requested [in this ticket](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/169).

 - Verified that the title when inserting links is "Insert link".
 - Verified that the title when updating links is "Update link".
 - The insert link dialog now displays the URL stored in the device's clipboard by default.
 - Added in a X to clear URL and title input.
 - The link icon now allows the edition of the link title.
 - Remove link button added in the update link dialog.
 - Reorganized some methods to be associated under the same pragma.
 - Documented some methods.
Technically wouldn't you want to use `NSParameterAssert`?