Improvements on main window in order to give more visibility to SQL tool
and central widget. Now there are three different views (welcome, design
and manage)
Version upgrade: 0.8.0-alpha2 -> 0.8.0-beta

Minor improvements on new main window arranjements.

Fixed the missing icons on modeldatabasediffform.ui
Minor improvement on pk constraints. Columns added to them will be
marked as not-null by default. This is done to avoid false-positive
changes on the model db diff process.
Merge branch 'mainwnd-improvements' into model-db-sync