Ubo-gui

- refactor: avoid loading kivy when constants are imported

- feat: widen the notification text horizontally if it doesn't have any actions on the left

- fix: updating items of the pages after the first page not being visible
- refactor: merge `HeaderMenuPageWidget` and `NormalMenuPageWidget` into `MenuPageWidget`
- refactor: keep a clone of menu-page widget as Kivy needs the source and target widgets to be different for transitions while going up and down in a menu needs a transition of the same widget to itself as we just change its items and don't recreate it

- fix: `go_home` should clear the selection of root before setting `stack`

- fix: going home from home caused a bad state

- feat: keep track of the selections, so that we can replace all the deep selections of an item when it's replaced