Automatic icon positioning

This commit introduces automatic icon positioning on a grid into kdesk.

You can turn it on by putting the following into the icon config:

Relative-to: grid
X: `grid-location-hint`
Y: `grid-location-hint`

kdesk will try to satisfy the field based on your location hints, but in cases it cannot, it will position the icon automatically. In case you don't care about the position of the icon at all, just set X and Y to 'auto'.

Note that the attributes Width and Height are IGNORED when the icon is put into the grid! Both of them are forced to 128, so the grid is uniform.

Related to: https://github.com/KanoComputing/peldins/issues/716

**TODO:** We might want to rename the attribute Relative-to to something different in the future.

cc @skarbat @alex5imon