Improved the mouse navigation tool

Add support for modifier keys, restore the relative zooming so that
it is useful for both large and small structures. Still need to take
care of zooming for orthographic projections.

Change-Id: Iaf3bc87057ade92ce297675c7835f467a9bfeddf
Tweak the initial zoom to offer better initial views

Change-Id: I7a96e7a88adde1b9006aaf4e17e7b568eba463a1
Refactor mouse operations, easier to use keys etc

Reduce code duplication, make it simpler to update actions.

Change-Id: I4fad06049bd06baef975046a4d0552aeeb2a249c
Add keyboard interaction for the navigator

Change-Id: I7fa63a9fe98168725b61a649db3e0bc1630461af
Merge branch 'improve-navigation'

Change-Id: I8ac7d3d3c3909a14b9209005ac26e7a82d39f1e0