* gptel.el (gptel--restore-backend, gptel--save-state,
gptel--restore-state): Move the Org-specific code for saving and
restoring state to gptel-org.
* gptel-org.el (gptel-org--entry-properties,
gptel-org--save-state, gptel-org--restore-state): Org-specific
code for saving and restoring state using Org properties, moved
from gptel-org.
* gptel.el (gptel--create-prompt): Split the Org mode logic in
`gptel--create-prompt` into gptel-org.el.
* gptel-org.el (gptel-org-branching-context,
gptel-org--create-prompt): Handle prompt creation for Org buffers
in `gptel-org--create-prompt`. The option
`gptel-org-branching-context` limits the context in Org buffers to
the direct ancestors of the active heading. This is useful for
conversations with branches. gptel-org.el is intended to
eventually contain all the Org mode specific gptel code, and will
not be loaded unless gptel is called in an Org buffer.
