Skip unnecessary up-/downloads of addressbooks when nothing has changed

Skip unnecessary up-/downloads of addressbooks when nothing has changed

- reimplement the pre-0.6.0 imap-upload policy and make it selectable via a global option
- added global option to be able to define addressbooks for up-/download separately

* Upload to IMAP folder should now work on Linux
* Added some menu items to the context and the tools menu in the addressbook window
* Added option to use POST instead of GET request when downloading via http(s)
* fixed bug with preference setting of external app handler
* When uploading to IMAP folder always delete old addressbooks from folder
* Single up-/download from the import/export tab in the settings now fully respects settings from the synchronize tab

- Added synchronization via IMAP folders
- Lots of bugs fixed

Fixed bug introduced in last-minute change in 0.4.1 :-(

fixed a bug with the masterpassword asked two times on startup