displaymanager: Detect the KDE Plasma (5 and 4) and GNOME desktop environments.

Add them to the front of the list of desktop environments because users
are more likely to expect those as the default if they have them
installed.

src/modules/displaymanager/main.py

@@ -27,6 +27,9 @@ import subprocess
 DesktopEnvironment = collections.namedtuple('DesktopEnvironment', ['executable', 'desktop_file'])
 
 desktop_environments = [
+    DesktopEnvironment('/usr/bin/startkde', 'plasma'), # KDE Plasma 5
+    DesktopEnvironment('/usr/bin/startkde', 'kde-plasma'), # KDE Plasma 4
+    DesktopEnvironment('/usr/bin/gnome-session', 'gnome'),
     DesktopEnvironment('/usr/bin/startxfce4', 'xfce'),
     DesktopEnvironment('/usr/bin/cinnamon-session', 'cinnamon-session'),
     DesktopEnvironment('/usr/bin/mate-session', 'mate'),