Update menu styling to use standard cinnamon classes

- Avoid cramped menu items
- Partially fixes #3
Force highlighted text to black

- Not ideal - we should get this value from the theme, but the default
  cinnamon themes don't define any sensible classes to inherit and at
  least you can read black on most colours (except black of course ;) )
- Partially fixes #3
Clear/set activities hint on menu toggle

- Completes set of fixes for #3