Improve adaptive UI
With the recent switch to libhandy the UI is now more usable on small displays, but still a bit too wide for mobile phones such as the PinePhone and Librem 5, which have 360px-wide displays.
This MR fine-tunes the UI so the window can be made to fit these devices' screens.
(please note that in Phosh we hide minimize/maximize/close buttons to achieve this)
Edited by Arnaud Ferraris