RFC: phosh-core: Make phosh-osk-stub the default OSK

The problems with key input of electron apps are unfixed since many years. Let's pick an OSK that hopefully can print letters for all the keys on a layout.

Marked as RFC as I want to see if we find convincing arguments in https://gitlab.gnome.org/World/Phosh/squeekboard/-/issues/425#note_2246668 or the matrix room to not do it.