keyboard: option to show/select extra layouts

[oprudkyi]

• add new checkbox "Include less-common layouts in the selections list"
  

• switch /org/mate/desktop/peripherals/keyboard/general/load-extra-items

• it is a port/re-work of linuxmint/cinnamon-control-center@d961517#diff-7525274c9dc606950625f46b7d8150e4223d7ad4eb9fae59d700d45539c4ccddR196

purpose of the feature.
it is really hard to know what option controls that extra layout list, I had spent hours groking why my laptop can show them and desktop can't
so probably it will be useful for newbies and such

P.S. please be extra careful with review - this is my first ever PR into gtk/mate code

[oprudkyi]

Hi @lukefromdc @oz123 @raveit65

Could you please check this

[lukefromdc]

I can verify from a test build that the checkbox works, but I have no way to tell if there is a difference in how many layouts are loaded as there are so many either way I cannot tell the difference.

Going to the code, I see the checkbox toggles the second argument to xkl_config_registry_load() but I have never worked on this part of the code so I really don't know how it works

Note that I know only one language and have never used a different keyboard layout myself. Therefore I have no idea which layouts would be less common.

A proper functional test of this requires knowing one layout that should appear only when the checkbox for less common layouts is checked. If that layout appears when and only when the checkbox is toggled, this PR can be presumed to work.
Any bugs would probably be issues in the existing code in libmatekbd we are toggling.

Also note that much of the team seems to be taking a break at this time, and it is beyond me to solo maintain all of MATE. We really need a review from someone who knows this part of the code.