Add tab_modified_indicator to the vim segments

This indicates in the tabline that any of the open buffers in a tab
page has unsaved modifications (like the default tabline do).

Signed-off-by: Jan Losinski <losinski@wh2.tu-dresden.de>
Remove useless TODO
Use spaces for alignment
Use tab_modified_indicator highlight group
Make tab_modified_indicator work in old Vims

Closes #996
Add tests
Always specify all keys in segment_info
Filter segments using mode in Theme.get_segments
Move processing segment['mode'] to segment.py
Merge pull request #1007 from ZyX-I/tab-modified-indicator

Add tab_modified indicator