[nemo-dbus] Add support for a{sv} type

[attah]

So, this adds rudimentary support fro the a{sv} type, which is array of string-variant pairs.
QT, sensibly, maps that to/from QVariantMap.

This was tested with harbour-seaprint-share-plugin calling org.freedesktop.Application.Open on harbour-seaprint from the SharePlugin QML.
(Commits there landing soon-ish, maybe on branch for the former)
It works both with and without content in the a{sv} object.

flattenVariantArrayGuessType looks fishy... but since it is done on plain variants, i do it here too.
I had some issues with it, but those were PEBKAC.

I have no idea how to run the tests, so i haven't done so, nor added any. Hints on that much appreciated.

[attah]

Having slept on it, org.freedesktop.Application.* is not actually needed for me and i can add whatever methods i please.
But all the same i think a{sv} variant map is useful, and being able to access those methods on arbitrary apps is neat.

[pvuorela]

I have no idea how to run the tests, so i haven't done so, nor added any. Hints on that much appreciated.

There's usually a -tests package with tests.xml. That can be used with testrunner-lite e.g.
"testrunner-lite -f /opt/tests/nemo-qml-plugins-qt5/dbus/test-definition/tests.xml -o dbus-test" which executes the tests and writes the output to a file. In this case, though, the tests includes only couple cases which are executed with the qmltestrunner so those commands could be executed manually as well.

The testrunner lite as a QA tool is not included in the main repositories, but it's in "tools". Suppose that could be still available if manually added with ssu ar etc. Sources in https://github.com/mer-tools/testrunner-lite

Besides that at least this appears good to me, though could do some test with the type still.

[attah]

Now with some basic tests.

Note to self: testrunner-lite does not tell you about how it went, qmltestrunner -input /opt/tests/nemo-qml-plugins-qt5/dbus/auto/tst_dbus_interface.qml, however, does.

[pvuorela]

Seems to do what it promises. Approved.

Might be nice for developers if we had documented the types supported, but that's an existing shortcoming.