Allow to select background themes for image maps

[GoogleCodeExporter]

Hi!
I would like to be able to select background themes as well for image maps.

This is for these reasons:
1) It would be nice to see how map X would look with background theme Y
2) If you un-select the default theme (which was a background theme) of a map, 
you can’t directly select it again, because background themes are usually 
hidden. You have to select the map again, which is slightly annoying. A map 
where this is the case is Eyes.

If you feel motivated, a checkbox would be nice to toggle the display of 
background themes on and off.

The term “background theme” is explained here:
http://hedgewars.org/node/5962

Original issue reported on code.google.com by almikes@aol.com on 1 Dec 2014 at 5:40

[GoogleCodeExporter]

So, by background theme, Wuzzy means themes that aren't complete/of low 
quality, so aren't in the normal theme list. Like Crazy Mission.

These could legitimately be selectable for image maps that don't rely on theme 
terrain/object placement (that is, if an image map has a map.png).

Would need to update icon appropriately when switching from a legit map to one 
that would be messed up.

Gonna go w/ this being low priority..

Original comment by kyberneticist@gmail.com on 1 Dec 2014 at 6:59

• Added labels: Priority-Low
• Removed labels: Priority-Medium