[#22] Fix duplicate rows / empty data on sorting.

Refactor so that child data not included in child count when
beginInsertRows is called. Necessary as beginInsertRows results in the
view calling rowCount etc to determine inserted rows.

Also remove redundant and potentially error causing parent option in
item constructor.
[#22] Remove redundant code in proxy model.
[#22] Ensure sort takes effect on model data change.
Merge pull request #23 from 4degrees/22_data_disappears_in_filebrowser_after_sorting

Fix #22: Data disappears in filebrowser after sorting