Implement node sorting in repository structure view

[lonely-lockley]

At this point all nodes are displayed in original order (as they were created). There's a need to separate storage from view by sorting nodes in tree grid first by their type and second - alphabetically. Sorting should be made this way:

1. directories first
2. all directories must be sorted alphabetically
3. all files last
4. all files must be sorted alphabetically

[lonely-lockley]

Completed in commit 0d98d0b