Menu registry should be flushed when switching themes

The blocks settings page displays the wrong default regions when you switch themes.

Steps to reproduce:

1. Make sure Bartik is the current theme
2. Go to Appearance and set Garland as default (Enable and set default)
3. Go to Structure > Blocks and see that Bartik is still the default tab

This is really strange, because a normal user who doesn't notice these tabs, drags and drops blocks to find out nothing happens on his site.

Flushing cache helps, so what we should do is rebuild the menu's after switching themes.
See the patch.