fix a font issue with qt4.x under yosemite
Merge pull request #328 from haiwen/fix_mac_font_issue_under_yosemite

fix a font issue with qt4.x under yosemite
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
add initial support for upload directory
copy task is assigned to another thread (global threadpool)