keyboard applet refactoring

[AxXxB]

A little more solid code, things like #9806, #8257 and #7792 should now never happen.

Also, the second commit adds a code that saves maximum width and height of applet (see #9505)

[Fantu]

@AxXxB thanks for your contribute, the keyboard layout will change significantly with the muffin rebase, so I suppose it's best to make the improvements directly there: linuxmint/muffin#601 (see also the related changes)
he will avoid wasting time with the rebase to you and / or mtwebster and maybe if you want you could help him for that part in the rebase
for faster/easy initial test/start can be useful my ppa with the 4 components updated for the rebase: https://launchpad.net/~fantonifabio/+archive/ubuntu/muffin-rebase/

[mtwebster]

Can you squash this down a bit? (Or I can merge into a single commit if you want)

Thanks

[AxXxB]

Single commit is fine =)

[Fantu]

hi, I did a fast test of muffin-5.2-test after a rebase including also this PR and I saw an error in logs: "Layout list is empty"
but seems applet still show layout list, I not searched deep in code but here a screenshot including also a trace that I suppose can be useful.

[mtwebster]

I think I fixed it, just pushed to that pr

[Fantu]

@mtwebster thanks, I did another fast test and I confirm is solved