Use correct .desktop file name when setting KMail as default

Summary:
Since Applications 17.08 KMail's desktop file is called org.kde.kmail2.desktop
in order to match its DBus name. This breaks setting KMail as a default
application, since org.kde.kmail.desktop no longer exists. This change checks
for existence of org.kde.kmail2.desktop file and falls back to using the
old name.

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D8239