Make windows not added to the space "always on top"

This avoids some ugly situations where dialog-type windows was very hard to
focus. Windows that lie below the tiling is usually not possible to click so one
would have to use alt-tab, the overview, etc..

Our own preference UI behaved like that :D (but only on X11)

Making them always on top isn't 100% ideal though - especially since transient
windows doesn't move when their parent move. (so they quickly end up above
other windows)

The search dialog of gedit is an example. Prior to this commit it would not stay
on top when gedit was moved away. It did "flash to the top" during animations
though..

Semirelated: #40

tiling.js

@@ -1747,6 +1747,8 @@ function insertWindow(metaWindow, {existing}) {
 
     if (!add_filter(metaWindow)) {
         connectSizeChanged();
+        // Make sure the window isn't hidden behind the space (eg. dialogs)
+        !existing && metaWindow.make_above()
         return;
     }