Dirs: Make string types nullable
Update POTFILES.in
tests/highlighting: Add hashtag test
Multiple nullable fixes
Remove AddFilterDialog

was never used anyway...
ITimeline: Add rerun_filters
MainWindow: Add rerun_filters

... which just calls rerun_filters on all the appropriate timelines,
i.e. just the MentionsTimeline and the HomeTimeline at the moment.
ModifyFilterDialog: Rerun filtes after changing a filter
TweetUtils: Only hide tweet if filter matches

We just hide it and add it to the timeline anyway, so we can show it
again later in case the user removes the matching filter(s).
Also rerun filters after deleting a filter
ITimeline: Remove old doc comments
DefaultTimeline: Handle invisible entries correctly when removing