Add 'has_parent' value to MtpcFileData.

Every file entry is set to having a parent by default. When the view reaches
the top-level folder of either the file-system or the device, we need to set
it to false.
Use the method 'mtpc_file_data_set_has_parent()' to set the property explicitly.
Cleanup 'mtpc_folder_tree_set_list()' method.

The mtpc_folder_tree_set_list() doesn't need 'parent_id' & 'current_path'
passed as arguments anymore. This method is a lot cleaner now.
Replace MtpcHomeFolderTree with MtpcFolderTree.
Remove mtpc-home-folder-tree.[ch].