Improve UiReport.

 * Add toolbutton to switch between errors of single file and errors of whole
   project. Rebuild list only if needed.
 * ReportWrapper: Also track deprecated elements and notes. Use two lists: One
   internal list to build up new results and one public list. This will keep
   the public list in consistent state.
 * Add new signal guanako_update_started and emit right before guanako update.
 * Add new (temporary) option --debuglevel. Currently valac errors have
   level 2.

 * Todo items:
    - Currently there is no difference between experimental and normal
      warnings. Could be difficult to map SourceReference to CodeNode (which
      is needed to get exp info).
    - Result may not be sorted (by lines or files). Set or Hash->Set may help.
    - Currently no gui element to filter (possibility is provided with flags
      enum).
    - Use -d/--debug with an optional argument and replace --debuglevel.

 * Problems:
    - Vala context update behaves very differently in producing errors when
      doing small changes in a single file.