Don't set tilda active when sub-windows are open.

I had some problems with the commit in 1bd3bbf.  Since I have tilda configured to take up most of the screen, when I right click and select "preferences", my mouse hits the tilda window after the preferences are created.  The preferences are then stuck below the tilda window and I can neither use them, nor use the pulldown key to get tilda out of the way!

I can't quite work out why tilda_window_set_active should be called at all on mouseover because the window should already be focused as soon as it is pulled down.

I'm a little unsure of what 1bd3bbf was intended to do so all I've done is revert the patch.  Perhaps the original author @aperkins81 could give some more information here?

Edit: reading closer I see this is to do with #18 .