EMailShellView cleanups.
EShellBackend cleanups.
Add em_folder_tree_ref_selected_store().

Replaces em_folder_tree_get_selected_store().

Same as before, but adds a reference to the returned CamelStore.
EMFolderTreeModel: Declare the CamelStore column as an object type.

Declaring the CamelStore column as a raw pointer type dates back to
before Camel was GObject-based.  The problem is the CamelStore could
be finalized while the tree model still has a pointer to it, leaving
behind dangling pointers in the tree model which could cause a crash.

Declaring the column type in the column enum turned out to be a good
idea, as it makes code that needs updated easy to find.  This renames
the enum value from COL_POINTER_CAMEL_STORE to COL_OBJECT_CAMEL_STORE.