Support for KDE!


![screenshot6](https://f.cloud.github.com/assets/1669594/106997/55285c1e-6a19-11e2-9ab3-cd745e338a26.png)
- Add a disabled menu item for displaying time only if running KDE,
  because it doesn't support indicator labels
- Use timedelta (not datetime) for calculating hours and minutes

Note: KDE doesn't seems to search icon in a custom theme path, so
icons need to be placed in a standard folder (/usr/share/pixmaps)