Editor: Blockselection rewrite.

Also adding the possibility to insert text into the blockselection.

Task-number: QTCREATORBUG-7773
Change-Id: I7a47a1d630f769a8253ee1a2f21057820ea170d5
Reviewed-by: Lukas Holecek <hluk@email.cz>
Reviewed-by: Christian Stenger <christian.stenger@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>