I'd love to get some feedback. :)
Please note I am not pretending this allows the window to reach small enough widths to fit a phone, nor this changes anything but the main window. This could come in later changes.
v2: adapted to libhandy-1
v3: dropped "non-main window" patches for now (I'll put them into separate MR w/ or w/out improvements, to keep them around) https://gitlab.gnome.org/okias/geary/-/commits/adaptive_dialogs