Called to allow custom display of the context menu. |params| provides
information about the context menu state. |model| contains the context
menu model resulting from OnBeforeContextMenu. For custom display return
true (1) and execute |callback| either synchronously or asynchronously
with the selected command ID. For default display return false (0). Do not
keep references to |params| or |model| outside of this callback.
Called to allow custom display of the context menu. |params| provides information about the context menu state. |model| contains the context menu model resulting from OnBeforeContextMenu. For custom display return true (1) and execute |callback| either synchronously or asynchronously with the selected command ID. For default display return false (0). Do not keep references to |params| or |model| outside of this callback.