Improve scrolling on addition on multicombo
Improve scrolling on addition on multicombo

* Make sure that the textiter will be valid when scrolling. Using an idle_add
  would sometimes use an already invalid iter

* Use PRIORITY_HIGH_IDLE + 25 instead of the default idle_add priority. The
  later one would be executed before the redrawing, which wouldn't allow us to
  scroll to some rows because they weren't yet drew.

Change-Id: Ieeedfc28a6f97d9331fd23bf1165472a2dfb44c8

stoq/kiwi