added gsettings for terminal's foreground and background color

I really love this terminal - simple, well integrated and not bloated. Great job so far, thanks!

The single thing i and some others (#33) missed is being able to change the background color to really fit to my themes. I saw that there was some code handling this but it was commented out. I activated it and soon discovered why you disabled it :) So i didn't touched it.

As you can see this patch just adds 2 settings to the gschema with hopefully reasonable defaults. This settings are dispatched to the terminal inside the existing handler  `DropDownTerminal._updateOpacityAndScrollbar` that handles transparency and which name didn't fit anymore. But i want my patch to be as slim as possible so i didn't renamed it.

As this is my first contribution on a gnome shell extension please tell me, if i missed anything. I just developed and tested this for 3.14. When you accept it i would add the gsettings command the the wiki page about the advanced configuration.