The `SpecialFolderType` was somewhat mis-named, since the special use
does not (in most cases) confer any special type, rather it's simply
defines what a particular folder is used /for/.
As such, rename the enum to to `Geary.Folder.SpecialUse`, moving it in
to the `Folder` class, since it relates specifically to folders, also
rename `Folder::special_folder_type` and
`Folder::special_folder_type_changed` reflect the above and not
duplicate the type name, and similarly rename
`Account::folders_special_type`.
Update the many call sites.
Instead, make the args property a parameter list, and add params to that.
This means Command's serialize method have a different signature compared
to Parameter's, letting us do some more interesting things with it.
* 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.
