Added button to hide DLC

[Logical-sh]

This adds a button to hide DLC.
This is a feature found in SteamEdit, and needed to play some games on linux (Assassin's creed II is an example. You need to hide the MAC key or it refuses to launch.)

[tralph3]

Nice one, I didn't notice this had other purposes than just hiding it from view. Just some notes, you don't need to cal update_launch_menu_window(), that function exists to... update the launch menu window (the one that lets you edit the launch menu for each game, so you can launch a different executable). It's called when you move options up/down or remove them, it basically creates the launch menu window from the info found on the dictionary. There's no need to call it, in fact, it's raising exceptions due to it.

Also, I would rather have the button always there, and enable/disable it depending on if the app is a dlc or not (I do something similar with the launch menu thing, I only enable it for things that make sense, a DLC ain't one of those things so it's greyed out. It's also disabled for uninstalled apps). Since you're at it, you can also check if the app is already hidden or not, and change the label accordingly. I'd much rather have this than packing/removing widgets on the fly, which can be messy.