Incremental sorting

[kirillt]

Right now, sorting of resources causes small delays on huge collections (e.g. 5000 of pictures). After addition of persistence of sorting preferences, this delay is imposed after frequent actions: opening a folder and going back from gallery mode to resources grid.

It may be possible to perform sorting in a smarter way:

1. find top T resources and put K of them into visible frame;
2. spawn sorting of the rest of resources in background;
3. allow the user to do his stuff.

Implementation can be non-trivial, but performance would be better:
blocking phase: T * N operations to find top T resources (where N is number of all resources)
background phase: normal sorting of N - T elements

[kirillt]

Eventually this should be implemented in the library, but it's not clear when it will be possible.

[kirillt]

Also, lazy shuffling (#15) could be implemented in similar fashion.