Add filters to the replay browser dialog

This closes issue #2152. The filters added are:

* Game type (singleplayer / multiplayer)
* Date
* Duration
* Outcome
* Player name

Other changes:

* Added a 'CollapseHiddenChildren' option to the ScrollPanelWidget to
make hidden children take up no space.
* Removed the extension (.rep) from the replay filenames in the
replay browser.

OpenRA.Game/FileFormats/ReplayMetadata.cs

@@ -53,6 +53,8 @@ namespace OpenRA.FileFormats
 			if (endGameTimestampUtc.Kind == DateTimeKind.Unspecified)
 				throw new ArgumentException("The 'Kind' property of the timestamp must be specified", "endGameTimestampUtc");
 			EndTimestampUtc = endGameTimestampUtc.ToUniversalTime();
+
+			Outcome = outcome;
 		}
 
 		ReplayMetadata(BinaryReader reader)