* moved universal 'TreeView' to utils
* use 'TreeView' in folders widget
* propagate 'set_deselectable' in 'FoldersWidget'
* propagate more public functionality of 'FoldersWidget'
* fix 'set_name_filer' typo
* rename 'get_current_project_name' to 'get_selected_project_name'
* added signals to task and project widgets
* implemented more helper methods in hierarchy model
* added more information to 'FolderItem'
* add empty line after docstring
* fix expected selection of folders in loader
* keep only 'double_clicked' signal
* pass full mouse event to signal
