apply minor improvements on the words of coments
add initial support for dropping file through repo tabs

- copy the dropped file to the worktree if the repository is synced
- add progressdialog for the mainwindow uploading
- don't bother the user if he (or she) cancels the uploading task
- support encrypted repos now
copy task is assigned to another thread (from global threadpool)
Merge pull request #319 from haiwen/drop_file_through_repo_tabs

add initial support for dropping file through repo tabs