subsequent taps on "Chats" should scroll up

[r10s]

in the chatlist, subsequent taps on the toolbar's "Chats" button currently does nothing.

in comparable apps, subsequent taps scroll up, this would be a little, nice improvement for Delta Chat as well.

to be clear: the first tap, should keep scroll position as it is now. there is no need to count taps or so - on a tap on "Chats", just check if "Chats" is already selected (scroll up) or not (switch to tab, use old scroll position as now)