Merge branch 'release-0.4' into development.
Merge branch 'hotfix-0.4.1' into development.
Automatically set the file extension when saving a grades listing.
Add icon for the export action.

In addition, it replaces the non-working "search" icon in the
toolbar.
URIs in labels can now be opened in an external browser.
Grades can now be exported just for one student group or for all.
Update Spanish and Galician translations for the latest changes.
Update Changelog and version number to 0.4.2.
Merge branch 'release-0.4.2'.