Even though proper size management is being worked on, this patch
proposes a simple and easily revertable solution to device-dependent
sizing issues.

First, it provides different calculations based on the display
orientation. In landscape mode, this allows us to have a sensible
keyboard size while leaving enough screen estate for apps to be able to
display useful information.

Then, it gets rid of the weird calculation for display widths between
360 and 540px. While having some continuity is a pleasant idea, in the
real world in doesn't work, as shown by port attempts to other devices:
a 480x800 display (scale 1) would show an unusable 190px-high keyboard
(about half the size of the Librem 5 on-screen keyboard on a device I
own).

Finally, this commit makes sure we never use a hardcoded size.

Tested on the PinePhone, PineTab and Librem 5.

Note: Current behavior is preserved on the L5 in portrait mode, but
keyboard is a bit smaller in landscape mode; this is deliberate, as it
was previously using too much space (causing some apps, such as chatty,
to be unusable).