Add new sync error string "Files are locked by other application".
add initial multi-selection supports

only download and remove actions are allowed on the multi-selection.
unselect the removed rows in FileTableView
chaneg background colors and its painting delegate
QStyledItemDelegate::paint should never be used
Merge pull request #355 from haiwen/change_background_colors

chaneg background colors and its painting delegate
Merge pull request #352 from haiwen/sync-err-str

Add new sync error string "Files are locked by other application".
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
try to open cached file if the file is cached previously