Added Combo control for DProperties

![prop_combo](https://cloud.githubusercontent.com/assets/3105615/3893078/d9b42c4c-2237-11e4-8259-1ca153a56d11.png)
Usage:
```lua
local DP = vgui.Create( "DProperties",  Panel )

local choice = DP:CreateRow( "Choices", "Combo #2: Custom default text" )
choice:Setup( "Combo", "Select type..." )
choice:AddChoice( "Table", {} )
choice:AddChoice( "Function", function()end )
choice:AddChoice( "String", "Hello world" )
choice.DataChanged = function( self, data )
	for k, v in pairs( data ) do
		print( k, v )
	end
end
```
I will make fully documented wiki pages if merged.