Adds participant filtering

This may prove particularly useful in case of very long user lists. Enables
the user to filter participants using the "starts with" logic.
Update variable name
Update the size request
Change filter behaviour from prefix to substring
Hide headers of participant list

They are redundant
Whitespace fixes
Try to follow GNOME HIG guidelines
Whitespaces

Remove tabs and replace them with blanks
More UI improvements
Try to get the toplevel window

This is overly conservative as it is unlikely that our widgets
are not inside a toplevel window.
Version bump
Fix missing translation
Update translations
Update copyright year
Port documentation from DocBook to Mallard
Rename image files
Minor improvement in the doc
Fix Makefile
Update PO