Add gui.group{arguments-gui.group.push, func}.

Convenience function that wraps func() in a push/pop pair, i.e.

	gui.group{grow=grow, spacing=spacing, size=size, pos=pos, function()
		do_stuff()
	end}

is equivalent to

	gui.group.push{grow=grow, spacing=spacing, size=size, pos=pos}
	do_stuff()
	gui.group.pop()

group.lua

@@ -138,6 +138,14 @@ return setmetatable({
 	beginFrame = beginFrame,
 	endFrame   = endFrame,
 	default    = default,
-}, {__index = function(_,k) 
+}, {
+	__index = function(_,k)
 		return ({size = current.size, pos = current.pos})[k]
-end})
+	end,
+	__call = function(_, info)
+		assert(type(info) == 'table' and type(info[1]) == 'function')
+		push(info)
+		info[1]()
+		pop()
+	end,
+})