Implement sidebar tree

This is very raw patch but very handy for projects. Currently it replaces old tree view without settings (I didn't find any settings about sidebar tree, only context menu).

Known bug: folding a dir also folds all nested dirs. Maybe this is GTK treeview feature... Another bug - after path splitting it doesn't merge parent path back while closing documents but it is concerned with recursive closing (touching tree while closing multiple documents will cause crash).

Screenshot (looks dirty if you open many files from different dirs but this is only demonstration):

http://pbrd.co/RbSSAp