Issue #3 - Don't launch preferences window as modal

This removes the `modal` parameter when launching the preferences window
from the Swarth toolbar button. While this works just fine on Windows,
it becomes a problem on *nix OSes since the color chooser window is not
opened as modal (this requires changes to the platform, see bug
1496836).

This resolves #3.

src/content/main.js

@@ -142,7 +142,7 @@ var Swarth = {
         openDialog(
             "chrome://swarth/content/preferences.xul",
             "_blank",
-            "chrome,modal,titlebar,centerscreen",
+            "chrome,titlebar,centerscreen",
             window
         );
     },