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.
* test/api/*.vala: Renamed files that contained mock objects to *-mock.vala,
not *-test.vala.
* test/testcase.vala: Renamed to test-case.vala for consistency, remove
TestCase class from Gee package since that's really not true. Clean up
code for consistency.
* test/meson.build, test/CMakeLists.txt: Split TestCase compilation out
into a separate test lib.
