Draft: Allow to screenshot applications to detect e.g. adaptiveness issues

Often one wants to know if certain pages of an app fit on mobile. With

meson setup -Dtests=true -Dtools=true _build
meson compile -C _build
_build/tools/check-adaptive tools/examples/phosh-mobile-settings.acheck

you can switch through the different pages and have them screenshoted by describing the navigation patterns like:

[app]
name = phosh-mobile-settings
exec = phosh-mobile-settings
quit = <ctrl>Q

[main]
shortcuts=

[welcome-tab]
shortcuts=Tab;Return;

[feedback-tab]
shortcuts=Return;Tab;Return;

[compositor-tab]
shortcuts=Return;Tab;Return;

[lockscreen-tab]
shortcuts=Return;Tab;Return;

[convergence-tab]
shortcuts=Return;Tab;Return;

Which gives you

This is marked as draft as I won't be able to look into cleaning this up for merging near tearm but the functionality might already be useful when you don't want to check your app manually every time.