WIP: language: Add support for click language packs

Created by: mardy

Experimental branch, do not merge yet.

This can be tested by making the image read-write (sudo mount -o rw,remount /), installing the new system-settings packages from Jenkins, then downloading a few clicks from https://ci.ubports.com/job/xenial-rootfs-minimal-armhf/24/artifact/clicks/ on the device and installing them:

pkcon  --allow-untrusted install-local com.ubports.language-fr_20181001-27507_all.click

Then, if you open the System Settings and go to the language page, you should see that you can only choose among the languages you installed. And if you confirm the new language and reboot, everything should work as expected.

Of course, the whole point of this effort is to get a smaller rootfs: this is available from https://ci.ubports.com/job/xenial-rootfs-minimal-armhf/lastSuccessfulBuild/artifact/out/ but I haven't yet verified if it works. :-)

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TODO

• Include in the clicks also the keyboard prediction files (/usr/share/maliit/plugins/com/ubuntu/lib/*): if we include only the databases, we can probably keep the .click files architecture independent; or we need to find a way to avoid the need for the binary .so files
• Add a "Install more languages" button to the System Settings
• Automatically upload the language clicks to the Open Store
• Update the Ubports installer to optionally use the new rootfs; and if we manage to find a way to preinstall the clicks onto the rootfs, we could even make the user select the desired languages from the installer