desktoppackage: fix title case in widget explorer

Widget explorer has a button that confirms uninstalling widgets. This button uses title case as recommended by the HIG, but does so incorrectly; it capitalizes the word "of" but short prepositions of less than four characters are supposed to be lower case in APA style (and most other style guides).

This change makes this label use correct title case and adds translation context.

before	after

String change, so 6.3 material

BUG: 492790 FIXED-IN: 6.3