Game Center/Guide overhaul for iOS 6+

Partially overhauled the Guide for iOS 6+ Game Center functionality:
Encapsulated repetitious code into single-purpose methods, fixed runtime
warnings for Game Center in iOS 7, and iOS 6+ no longer uses deprecated
methods to open Game Center windows (all compiler warnings in Guide.cs
are fixed). Also fixed a MatchMaker crash bug. Also fixed the formatting
(tabs, spaces, braces) in Guide.cs to be in line with MonoGame's coding
conventions. Bonus feature: Added access to iOS's Twitter API to the
Guide now too. Tested on iOS 5, 6, and 7.
More robust handling to platforms in EffectProcessor.
Added exception for PS4 case.
Added initial flicker free tree updating support.
You can now add multiple files at once.
Added Edit menu.
Delete key now deletes the active item.
Tree items are keep in alphabetical order.
Picked better font for output window.
Merge pull request #2508 from SickheadGames/pipeline3

More Pipeline GUI Improvements
Merge branch 'develop' of https://github.com/mono/MonoGame into develop

Conflicts:
	MonoGame.Framework/iOS/GamerServices/Guide.cs
Merge pull request #2504 from picobots/develop

Game Center/Guide overhaul for iOS 6+
Hooked up about and help items to website.
Fixed missing continue.
Added "Add Item" and "New Item" to the edit menu.
Added Total Items stats to project.
Added property grid attributes to project object.
Added property grid attributes to content and folder objects.
Fixed title of new project saving dialog.
We no longer close the project before creating the new one.
You can now cancel the content build.
Merge pull request #2509 from SickheadGames/pipeline4

More Pipeline GUI Improvements - Part 2