Update string resources.
Fix #311 - AvalonEdit cannot be used in WPF application with multiple UI threads.
Handle errors when saving the code completion usage cache.
StorageLocationPicker: fix switching from a configuration-specific location to the base location.

The CoerceValue-callback is not called by WPF for values set by the control itself, so it was setting ProjectProperty.Location to PropertyStorageLocations.Unchanged instead of PropertyStorageLocations.Base.