dropbox: fix tray menu with StatusNotifierItem trays

If dropbox can't load libappindicator, it will fallback to using the old
xembed protocol. If you (or your Desktop Environment) are running
something like KDE's XEmbed SNI Proxy, then you'll at least see an icon,
but clicking the icon won't give you menu.

Adding libappindicator to the env allows dropbox to fully integrate with
contemporary status trays, while still falling back to xembed if the
required DBus service is not present.

pkgs/applications/networking/dropbox/default.nix

@@ -36,7 +36,7 @@ buildFHSUserEnv {
     libICE libSM libX11 libXcomposite libXdamage libXext libXfixes libXrender
     libXxf86vm libxcb xkeyboardconfig
     curl dbus firefox-bin fontconfig freetype gcc glib gnutar libxml2 libxslt
-    procps zlib mesa libxshmfence libpthreadstubs
+    procps zlib mesa libxshmfence libpthreadstubs libappindicator
   ];
 
   extraInstallCommands = ''