Add em_folder_tree_model_get_row_reference().

Replaces em_folder_tree_model_lookup_uri() and to some extent
em_folder_tree_model_lookup_store_info().

Working toward making the EMFolderTreeModelStoreInfo struct private to
EMFolderTreeModel, so it can then be made thread-safe.
Remove em_folder_tree_model_lookup_uri().

No longer used.
Change em_folder_tree_model_set_folder_info() argument.

Take a CamelStore instead of an EMFolderTreeModelStoreInfo.

It imposes a few extra hash table lookups to reacquire the StoreInfo
from a CamelStore while populating the folder tree, but that's fine.

Working toward making the EMFolderTreeModelStoreInfo struct private to
EMFolderTreeModel, so it can then be made thread-safe.
Make em_folder_tree_model_remove_folders() private.

Only called from within EMFolderTreeModel itself.

Working toward making the EMFolderTreeModelStoreInfo struct private to
EMFolderTreeModel, so it can then be made thread-safe.
Make em_folder_tree_model_lookup_store_info() private.

Only called from within EMFolderTreeModel itself.

Working toward making the EMFolderTreeModelStoreInfo struct private to
EMFolderTreeModel, so it can then be made thread-safe.
Make EMFolderTreeModelStoreInfo private.

Also shorten the struct name to StoreInfo since it's private now.
EMFolderTreeModel: Give StoreInfo a reference count.
EMFolderTreeModel: Thread-safety improvements.