Set StartupWMClass in desktop file

Used by non-Plasma unixoid X-based shells to map windows to desktop files
e.g. in launcher UIs, which do not know about _KDE_NET_WM_DESKTOP_FILE and
instead expect (one of) the WM_CLASS entries to match the basename of the
desktop file by default. StartupWMClass overwrites that default.

app/org.kde.massif-visualizer.desktop

@@ -7,6 +7,7 @@ MimeType=application/x-valgrind-massif;
 Icon=massif-visualizer
 # FIXME: write docs!
 # X-DocPath=kcachegrind/index.html
+StartupWMClass=massif-visualizer
 Terminal=false
 Name=Massif-Visualizer
 Name[bs]=Massif-Visualizer