Codepaster: Join actions, always open dialog.

Remove the "Paste from clipboard" action and move the functionality
into "Paste Snippet". Introduce a new overload for post() taking
a flag mask specifying the sources and use this for the
CodePasterService class (used by the diff editor).

Task-number: QTCREATORBUG-13401
Change-Id: Iadc3560a8a3bdaa817bc4a86007c2682feeb4b77
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>