Sidebar: Make the disk indicators theme-controlled for various properties

- Max length
- Foreground color
- Background color
- Thickness
- Corner radius
- Extra padding from bottom of cell

see src/nemo-style-fallback.css
Set some sane default values for disk indicator styles.
Use a custom GtkTreeView class to hold the style properties.
Sidebar disk indicators: Handle bad styles better, fix a couple problems
Merge pull request #196 from mtwebster/theme-disk-bars

Sidebar: Make the disk indicators theme-controlled for various properties