Fix adding and editing categories and activities; translation stuff

This fixes a crash I get when editing an activity or category using the mouse or the keyboard. The traceback, actually one of four, is:

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/hamster/preferences.py", line 491, in on_category_edit_clicked
    self.category_tree.set_cursor_on_cell(path, focus_column = self.categoryColumn, start_editing = True)
TypeError: set_cursor_on_cell() takes exactly 5 non-keyword arguments (2 given)

Adding a category I got this:
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/hamster/preferences.py", line 547, in on_category_add_clicked
    start_editing = True)
TypeError: argument path: Expected Gtk.TreePath, but got tuple

And making some strings translatable and update Potfiles.in.

I hope the git log isn't too messed up, I reverted a commit and merged master, didn't think this would be visible in the pull request.