FIXES #1212

 - AppData and AppStream can be disabled independently of finding
   their requirements (possibly useful if you want to ignore
   AppStream even when it's installed in your build environment).
 - Add a little top-level documentation about WITH_

 - Make it easier to obtain locale-ids (from CALAMARES_TRANSLATION_LANGUAGES)
   so avoid splitting that string multiple times.

 - Don't build a Pool for each PackageItem loaded
 - Do make it load all languages instead of only the current one

 - These don't have to be static methods of PackageItem, a free
   function is more convenient.
 - Since it's not API of PackageItem anymore, need to
   - update tests not to use API
   - do API-not-available warnings in consumers

 - The smallest size image of the default (or, if there is no
   default, the first) screenshot is used.
 - Remote URLs are not supported by QPixmap, so most will not
   load anyway.

 - Get the id, name, and description from AppStream data

Missing:
 - No translations
 - No screenshots

 - Use *appstream* as key in one of the items for the package-
   chooser to load data from the AppStream cache in the system.
 - Usable for some applications; for DE-selection not so much.
 - Currently unimplemented.

 - Put the implementation entirely in a separate file, keep the
   not-supported one in PackageModel.cpp (but only in an #ifdef).
 - Makes the various optional-data-sources more similar.

 - Simplify the iteration by first determining which partitions
   are mountable (at all).
 - This guards against the very rare case that a partition
   does not have a mountPoint at all (the if guarded against that)
   where the lambda passed to sort() would get a KeyError.

mount: Make sure extra mounts are mounted right after /

Instead of having a special case for extra mounts to be processed right
after the rootfs, a better approach is to add them to the partitions
list, and then sort the list by mount point.

This way, we make sure every partition is mounted right when it is
needed: `/` is obviously mounted first, `/run` is mounted before
`/run/udev`, and so on.

The overall process is therefore more generic and should suit all
use-cases.
Signed-off-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>

 - Improves the settings in welcome.conf, allows setting URLs directly
 - Adds a *showDonateUrl* setting to add a donations-button

 - the "Select language" tooltip was applied to the form, so it would
   show up inappropriately all over the place
 - the buttons didn't have useful tooltips.

 - having show*Url and donateUrl seems inconsistent, although
   the show*Url settings were originally boolean-only.
 - add "show" to the Donate button setting, to make them
   all consistent (putting a boolean there will generate a
   warning and hide the button, that's all).

 - the show* settings in welcome.conf are "upgraded" to allow
   setting the URL directly in the module configuration.

 - the generic (enum-based) setupButton() can handle all four
   of the buttons, so setupLinks() can go away. Only the
   (re)translation of the text on the button needs to be
   done, so move that to the main RETRANSLATE.

 - Handle buttons and their URL-opening in a more
   general way with an enum; drop existing three-boot
   method and special setupDonateButton()
 - Doesn't compile because consumers haven't changed.

FIXES #1197

 - LGPLv3 like the others

 - .. or, rather, avoid re-formatting tables in future.

Add 'base 10' partition size multiples