SettingsDialog: Save geometry

Since other windows do it, we should also do it for the settings dialog.
Closes #102
TweetUtils: Correctly escape tooltip text

For some reason Gtk+ needs a doubly-escaped ampersand here.
Tweet: Correct indentation for debugging Json.Generator
tweet-list-entry: Don't forget about rt_label's link-activated signal

Fixes issue #107.
Minor coding style
SettingsDialog: Remove unused variable
ImageDialog: Correctly close the save dialog on cancel
Add manpage
data/CMakeLists.txt: Install manpage to $prefix/share/man/man1
TweetUtils: Don't recreate Soup.Sessions all the time

This kinda fixes the crash we saw on current Fedora rawhide and also
on Ubuntu 14.04.

Related to #105 and https://bugzilla.redhat.com/show_bug.cgi?id=1061571
FavoritesTimeline: Just unfavorite the tweet when unfavoriting

Yes, really.
Related to -- and maybe even fixing -- #103
ComposeTweetWindow: Also add all mentioned user when replying

This is the 'normal' behavior. See isseue #97
ComposeTweetWindow: Don't forget the space after the last mention

Forgot that in c434ec29542fb26b779635b8fbf8335a72c52737
Tweet: Filter mentions more strictly

We just ignore multiple mentions of the same user as well as when the
authenticated user get mentioned.
That unfortunately makes it necessary to pass the used account instance
to `load_from_json`.

See issue #97.
TweetUtils: Use array in work_array

No need to use a GList here.
manpage: Add --tweet parameter

It exists, even though it does not come up in --help for some reason.
gdk pixbuf vapi: Remove _from_stream_async functions

We redefine them in our code anyway.
FavoritesTimeline: Remove unfavorited tweets on_leave

See #103
UserStream: Don't identifty status_withheld events as delete events
Merge branch 'master' into continuous

Conflicts:
	src/util/TweetUtils.vala