add initial multi-selection supports

only download and remove actions are allowed on the multi-selection.
unselect the removed rows in FileTableView
handle cancel event correctly with PostFilesTask
Merge pull request #357 from haiwen/handle_cancel_event_correctly

handle cancel event correctly with PostFilesTask
Merge pull request #343 from haiwen/feature_multi-selectable_file_browser

[RFC] add initial multi-selection supports
free all GErrors returned from libsearpc
free the components of SeafileApplet when the program exits