added an appdata file for corebird.
ImageDialog: Fix save dialog

Fixes #29
ImageDialog: Fix valac warning
Merge pull request #33 from ryanlerch/appdata

added an appdata file for corebird.
MainWindow/switch_page: If cur_page == page_id, just call on_join

... it won't be accessible via the back button, but it's still better
than our current approach of just ignoring it.
Merge branch 'master' of https://github.com/baedert/corebird