libhawaiishell: Add busy property to Element

Elements can set busy to true when they are doing something.
Views will show a busy indicator on top of busy elements.

Issue: #122
libhawaiishell: Add immutable property to QuickView

Immutable views cannot be configured by the user, by default views are
mutable.

Issue: #122
desktop: Configure panels with drag & drop

Panels can now be configured with drag & drop if not immutable.

Issue: #122
desktop: Prepare desktop view to receive drop

Issue: #122
indicators: Fix alignment

Issue: #122