Import standard GNOME Flatpak CI config rather than copy/pasting it

Closes #187

See merge request !87

Update Gitlab CI Flatpak build with standard template

Closes #187

See merge request !86

Only collapse runs of space, tab, carriage return, and newline

Closes #184

See merge request !83

Do not add childless, non openable folders entry in sidemenu

See merge request !84

See: https://gitlab.gnome.org/GNOME/citemplates

Closes #184

Fix special folder duplicates

Closes #181

See merge request !82

This time, it was overriding SPECIAL-USE attrs based on the config file,
causing bad config folders to be used as GMail specials. Now, it checks
the folder is already set as the special type before going looking for
it by name.

Convert getters that look like properties into actual properties,
remove unused and redundant public and internal API, convert
implementation to use a tree that aucyally maintains references between
steps, rather than each creating a new list of path steps and
manipulating that.

Instead of each top-level IMAP folder being a FolderRoot object, then
children of that being FolderPath objects, this makes FolderRoot an
"empty" FolderPath, so that both top-level and descendant folders are
plain FolderPath objects. Aside from being more technically correct,
this means that empty namespace roots can now be used interchangably
with non-empty namespace roots (addressing issue #181), and custom
folder implementations no longer need to provide their own trivial,
custom FolderRoot.

To support this, a notion of an IMAP root and a local root have been
added from which all remote and local folder paths are now derived,
existing places that assume top-level == root have been fixed, and
unit tests have been added.

Since duplicates are being cleand up on startup, this ensures dups
never exist in the DB.

Minimal folder should force a folder closed when an error occurrs that
it isn't going to be able to recover from.

Meson build fixes for FreeBSD

See merge request !78

Misc build warning fixes

See merge request !81

PropertyCommand: check for null before calling printf

See merge request !79

Support TLS certificate pinning when GCR can't find usable PKCS#11 slots

Closes #20

See merge request !80

This re-adds support for using GCR for pinning certs, but only if GCR
is in a known good state. If so, pinned certs will be stored using GCR,
if not, they will be stored in Geary's XDG data dirs as a fallback.

Rather than store a list of pinned certs each with a set of idenities,
just use a map of idenities to certs. This simplifies both storage,
lookup, retreival, and will allow pins to be persisted on a per-identity
bassis, allowing a cert to be saved for one host but not saved for
another.

Move common code for prompting for cert errors to the account editor,
use that when validating an account from the server pane.