Remove PoeditFrame:: m_itemsRefreshQueue.

This idle-time refreshing items code was needlessly complicated,
esentially doing what wxListCtrl::RefreshItem() already did.  Written in
2008, the original reason for it is apparently long obsolete.
Add some operations to Catalog class itself.

Add ClearTranslation() and SetTranslationFromSource() methods into the
Catalog class, so that they can be performed directly on the data rather
than on the UI controls for the currently displayed item.
Implement multiple selection.

Make it possible to select multiple items in the list and perform
operations on all of them.  In particular, it is now possible to (un)set
fuzzy status on many items at once, as well as clear or copy the
translation or edit the comment.
Merge branch 'multiple-selection'