Folder: track icon movements across screen resolutions

Positioner: track icon movements across screen resolutions

Track icon movement (e.g. by drag and drop) in current screen resolution
and apply them when resolution changes as long as positions don't
overlap and it fits on the screen without scroll bar.

This makes possible to have consistent icon layout when changing screen
scaling or changing monitors.

When changing to non-existent resolution, the scroll bar may appear.

Positioner: use hash table for positions instead of string list

It allows for fast and easy key lookup, also we don't have to convert
string to integer to read grid position.

Also the code is cleaner and helper functions are no longer needed.