Tweaked some visual styles for box rendering (Fixes #4)

- Turned on Z-indexing for selected boxes.
- Improved hover behaviour / SELECTED behaviour so that tag preview link line shows more consistently
- Added a hovering state to the model
- Got the tag preview to hide its border (underline) and ensured it only showed when linked annotation was hovered over / selected
Merge pull request #10 from QutBioacoustics/boxvisualstates

Tweaked some visual styles for box rendering (Fixes #4)