[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