Visual & performance improvements, new background

* Visual improvements:
- Items have a few pixels distance from top and left screen edges.
- Item descriptions, selection highlighting, and the circle menu all
have received some form of shadow or background to increase contrast on
certain backgrounds.
- Files originating from multiple different locations (Windows: user
desktop & public desktop) are all sorted together alphabetically. No
differences.

* The performance was improved a lot -- mainly not for Jesk, but for the
rest of the computer. When not used, Jesk stays relatively quiet now,
not using a lot of the CPU (which it did before). Only when interaction
requires it more update ticks are performed.

* A new background image was added, replacing the previous placeholder
kind of picture.