Change mainmenu texture handling + small misc changes

Texture names must now be escaped in formspec elements image[],
background[], image_button[], image_button_exit[].

Instead of special-case handling of texture loading (and unloading
which was missing) in guiFormSpecMenu.cpp, use the newly created
ISimpleTextureSource interface which is a minimal subset of
ITextureSource. There is an implementation of this interface
used by GUIEngine (MenuTextureSource).

Fix an off-by-one bug in unescape_string; it caused requests for a
texture called "\0".

builtin/modstore.lua

@@ -227,7 +227,7 @@ function modstore.getmodlist(list)
 			end
 			
 			retval = retval .. "image[0,".. screenshot_ypos .. ";3,2;" .. 
-					list.data[i].texturename .. "]"
+					engine.formspec_escape(list.data[i].texturename) .. "]"
 			
 			--title + author
 			retval = retval .."label[2.75," .. screenshot_ypos .. ";" ..