Don't center the room list on clicks by default

Previously, clicking to select a room would make the list jump around
to become centered again. This behavior is now reserved to keyboard
navigation.

A "centerRoomListOnClick" setting has been added to allow going back
to the previous behavior.

src/backend/user_files.py

@@ -269,6 +269,7 @@ class UISettings(JSONDataFile):
             "alwaysCenterRoomHeader": False,
             # "autoHideScrollBarsAfterMsec": 2000,
             "beUnavailableAfterSecondsIdle": 60 * 10,
+            "centerRoomListOnClick": False,
             "compactMode": False,
             "clearRoomFilterOnEnter": True,
             "clearRoomFilterOnEscape": True,