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
reverse using ServerRepo as FileNetworkTask ctor param
accept the drop event on repo tree view
Update version to 4.0.3.
use transfer mgr to manage upload/download tasks
WIP: display download progress in file table