update notes for making a new release (add RCs, adjust to Github changes)
reduce memory leak in GtkTreeStore

Looks like there's a memory leak when using Gtk.TreeStore.append() with a
list of initial values. I've seen this at least on Ubuntu 12.04. Might be
bugs.gnome.org/693402 .

Even with this workaround, resident memory usage doesn't go down when
closing a result panel; but at least that memory is reused when starting
a new search. Also there appear to be other memory leaks in pygobject
under Ubuntu 12.04, which are fixed eg. in Fedora 19.

Test:
- start Gedit, and watch its RES memory usage in "top"
- start a huge search (eg. search for "a" in /usr/share)
- stop search when ca. 300.000 results have been added
- note RES memory usage of gedit process
- close result pane
- start same search again, and cancel it at ca. 300.000 results again
- compare current RES memory usage with previous value
  - good: memory usage has increased by max. 5MB
  - bad: memory usage has increased by more than 20MB
move FileSearcher class into new result_panel.py file
rename FileSearcher class to ResultPanel
reorder and re-group functions in ResultPanel a bit
move search dialog into new search_dialog.py file
lazily create SearchDialog object (when it is first used)

This should slightly reduce Gedit startup time, and should reduce memory
usage if plugin is never actually used in a Gedit window.
reorder and regroup functions in search_dialog.py
reorder and regroup functions in ui.py
rename ui.py to plugin.py
add new .py files to Makefile
move common translation stuff into new plugin_common.py file
move resource path lookup into plugin_common.py file