Fixes #407 on stable branch

This was returning prebuild, causing prebuild to run twice.

This is calling the wrong vfunc to finish. Generally, it will be fine since
everything is using GTask with booleans anyway, but still.

This backports various fixes from master to remove threading from the
configuration initialization as well as some other cleanup.

Backported from #353

No need to imply the sink to the transfer of the call, so hoist this out.

Backported from master

Ensuring the ownership with items in multiple queues is just too burdensome
for a use-after-free scenario. So instead, always keep a copy of the
CXCursor.

We've already done something like this in the code-index revamp on master,
so this is only a backport.

mesonintrospect was failing to execute properly due to an
internal meson error. This makes things work similar to how
it works for us on master (with the C port of meson plugin).

Fixes #257

We want to access these from the main thread to get a snapshot of the
modified buffers.

This should probably be refactored before long to properly use cancellables
across the async function boundary. But for now, just track once we've
unloaded and avoid doing work in those conditions.

This fixes a gtk_widget_destroy() warning after unloading.

In case someone has a new libdazzle but old builder, handle the ABI break
that was done in libdazzle.

This backports some work to make the usage of weak references to the buffer
safer in presence of unexpected buffer finalization.

We might get a valid tree pointer back, but it contains no
children nodes. In this case, simply ignore the tree and
instead try the next symbol resolver who might have something
more interesting.

This tasks runs asynchronously, so we need to ensure that we keep the
resolvers alive for the lifetime of the operation.

We need to access private data after chaining up, so ensure we
have a reference to make that safe.