fractal-gtk: make input line look clean and aesthetical

With OSX-Arc-White before patch:

(feel free to ignore icon from !553 (merged) result is same).

and now this is how line looks after fix:

as you can see buttons are slightly bigger (better on touchscreens) and most important aspect is, that they're in line with input field! As a bonus, it nicely scales when input field turns multi-line (again, extra bonus for touchscreens)

Thank you for attention :)

EDIT: Screenshot is done with "OSX-Arc-White" theme, BUT this is correct behaviour, while when using Adwaita and input scales to two or three lines, it's ugly anyway :)

With Adwaita before:

Adwaita after:

Experiment with baseline: