add tooltips

[futurepaul]

tooltips are a combination of state and custom widgetry:

they need to be controlled by some sort of global state that keeps track if they've been shown yet

there's a tooltip widget in flutter but it's not customizable enough to work for our purposes I believe. will need a way of positioning without breaking the existing layout

I started on the visual of this widget here: https://github.com/futurepaul/fluttermint/blob/master/lib/widgets/fedi_tooltip.dart