Add new API for the leaks tracer plugin

commit b126c76d:

leakstracer: Improve documentation for the element
Also print a useful g_warning() message when leaks are detected.

commit 6c9b1859:

tracers: Allow setting a name for all tracer objects
This will be useful in combination with the next commit when we add
API to get a list of active tracers so that consumers of the API can
easily distinguish tracer objects.

commit 371dd624:

gsttracer: Add new API to fetch the list of active tracers
This will be useful in the next commit where we add action-signals on
the leaks tracer to get information about leaks and to manipulate
checkpoints as a replacement for the SIGUSR1 and SIGUSR2 signals for
doing the same.

commit 0480f9c1:

leakstracer: Add API for fetching leaked objects
This allows programs to inspect the leaked objects directly, log them,
and so on. Unlike the existing mechanism to use SIGUSR1, this also
works on platforms that do not support UNIX signals, such as Windows
and with applications that already use SIGUSR1 for something else.

commit 761fb823:

leakstracer: Add API for logging leaks in the debug log
This is the equivalent of sending SIGUSR1 to the application, and is
useful on platforms where UNIX signals are not available, such as
Windows.

commit 99e317cf:

tests: Add test for new live-objects leaktracer API

commit 7e941d4a:

leakstracer: Add API for tracking and checkpointing objects
This feature was previously available only through the SIGUSR2 signal,
which meant it wasn't available on platforms that don't have UNIX
signals, such as Windows and with applications that already use
SIGUSR1 for something else.

Now we have action-signals for doing the same. These action signals
can also be used for fetching the checkpoint information
programmatically instead of printing to the debug log.

commit 917bb43a:

tests: Add test for new activity-tracking leaktracer API