SSH Tunneling and advanced bookmark editing

[bk138]

Closes #102

TODO

• [x] decide what to show on New Connection UI
• [x] find a better alternative to those Spinners and enums? -> no, keep this for now
• [x] load and display ssh stuff from bookmarks
• [x] decide if the keep option makes sense -> no, remove!
• [x] wait for https://github.com/libssh2/libssh2/pull/712 or directly use https://github.com/bk138/libssh2
• [x] test test test!
• [x] update translation strings
• [ ] check db migration
• [x] fix bookmark name dialog layout
• [x] add bookmark name dialog when saving active conn
• [x] show encryption status in VncCanvas