Add SQL migration that drops the old FTS4 MessageSearchTable table,
re-create as a FTS5 table, clean up the column names a bit, and adds a
flags column so unread/starred queries can be made fast.
Define a SQLite FTS5 extension function `geary_matches()` to replace
the FTS3 `offsets()` function which no longer exists in FTS5, based on
Tracker's implementation.
Update code to FTS5 conventions (docid -> rowid, etc), use new column
names, populate and update the flags column as the email's flags
change, and use new match function for getting matching tokens.
Advanced searches are probably currently broken, these will be fixed
by subsequent commits.
Some remarks:
* Note that Meson adds a hard dependency on Python 3.
* All dependencies and defines are now listed together.
* Some build files were put in their respective subdirectories, e.g. the Geary
engine library will be built from the Meson file in `src/engine`.
* `--fatal-warnings` is no longer an explicit flag, as Meson provides
`-Dwerror=true` for this.
* An explicit resource file needs to be used. The issue to support this from
Meson itself can be found at https://github.com/mesonbuild/meson/issues/706 .
* The `gnome.yelp()` function parses a LINGUAS file so we no longer need to keep
track of all languages in our build system.
* There are no Debian scripts defined in the meson.build files to keep them
clean, but they can be kept as separate scripts in `build-aux`.
* Left out the `dist` target as there is now `ninja dist`
* `geary-docs` is disabled by default, as valadoc-0.38.3 returns errors.
https://bugzilla.gnome.org/show_bug.cgi?id=777044
