0003207: Various small code reworks/perf tweaks (GUIControlFactory, GUIWindowManager, (1 Viewer)

[MediaPortal-Bot]

Made some fields read-only where safe/recommended by Resharper in various classes, especially lock-objects.
GUIControlFactory tuned a bit
Simplified GUIWindowManager away from array-index to simple dictionary, which should also be a bit faster, but not noticibly probably.
Optimized Texturepacker.Get to use far less string comparisons

http://mantis.team-mediaportal.com/view.php?id=3207

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