Move user directory detection location to UICommon.

The UI should decide on where it wants the user directory, not our core system.
This is in anticipation of some upcoming work on Android which will need proper user directory setting.

Source/Android/src/org/dolphinemu/dolphinemu/NativeLibrary.java

@@ -169,6 +169,12 @@ public final class NativeLibrary
 	 */
 	public static native void CreateUserFolders();
 
+	/**
+	 * Sets the current working user directory
+	 * If not set, it auto-detects a location
+	 */
+	public static native void SetUserDirectory(String directory);
+
 	/**
 	 * Begins emulation.
 	 *