Exposing testing harness/framework to other libweston-based compositors

In short this series installs the headers (fixture, client helpers and the testsuite-data) and the static libraries to allow other compositors re-use the testing harness for writing specific tests for libweston-based compositors.

It adds a test-dev-install meson option as a build-time option to avoid installing it by default.

This series:

• installs the libweston-test and libweston-test-runner static libraries which tests can link to
• install test-plugin.so shared library needed to create the idle task to actually start the test
• installs the headers (client helpers, compositor fixture and testsuite-data headers)
• removes dependencies from headers and adds them in the test so they can be included freely from other tests
• makes public some of the objects that were in testharness_private
• adds the ability to use a different screenshooter as weston_screenshooter is a private extension to weston
• adds a way to pass additional arguments to the compositor instance and removes the shell option in case the compositor doesn't load one
• adds test-dev-install meson option to install all the above only when specified

With these added I can successfully link and write tests out-of-tree and re-use the client helpers to bind to a different screenshooter extension and point it to a different screenshooter function.