[notifications] Read appname from desktop file if no appname set

Summary: Some applications, e.g. pamac, don't send an app_name, which is legal according to the fd.o spec. In this case try to read it from the desktop file before falling back to the process name.

Test Plan: I get a better appname for pamac notifications now

Reviewers: #plasma, broulik, mart

Reviewed By: #plasma, mart

Subscribers: crossi, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27786