gtk4 a11y: Handle accessible relations

When creating a new LoAccessible, also consider
accessible relations.
These can be retrieved via the XAccessibleRelationSet
UNO interface.

Bridge those that have an equivalent in the GTK 4 a11y API.

As the parent GtkAccessible of a target XAccessible
is not known, extend `GtkAccessibleRegistry::getLOAccessible`
with the functionality to find a parent via the XAccessible
hierarchy (i.e. by walking up the XAccessible hierarchy
until a parent is reached that has an entry in registry
already).
This could potentially be problematic as an a11y hierarchy
solely based on the XAccessible hierarchy doesn't necessarily
match the one with the GtkAccessibles from the native GtkWidget
hierarchy. In my current understanding, this should presumably
be OK as long as that mechanism finds one existing
entry before getting to the root, so assert that this is the case.

With this in place, having a Writer doc with multiple paragraphs,
there is a flows_to relation shown for the first paragraph to the
second in Accerciser, etc., and jumping to the target selects
the next paragraph as expected.

Change-Id: I2ec1fe8c48dd4250d407ae4fb3c8d9c0f6671646
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/166926
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

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