added refresh-list feature

- when the Buildmenu has focus and the user presses 'R', then
  command './waf list' is issued again and the list of
  build-targets is refreshed (markings of targets are discarded)
- command './waf list' is only executed once when Buildmenu
  is opened for the first time in a vim session. in commits before
  9a5efb0 './waf list' was executed each time when Buildmenu
  was (Re-)opened. On big code projects execution of this command
  can take a few seconds, so the current solution should make
  Buildmenu more responsive