add some actions for context menu
if cancelled, don't continue to do the rename task
rename RenameDirent* to RenameFolder* and trim the input of new_name
refine the words of confirmation

discussion: https://github.com/haiwen/seafile-client/pull/258#discussion_r19922023
Merge pull request #258 from Chilledheart/patch-14

add a right click menu for item