Stores the currently open (right-click) menu in MyTreeView
and adds a `close_menu()` method so inheritants can cleanly
close the menu again if the context it was opened upon has changed.
This is utilized by `AddressList` and `UTXOList` to close the menu
if a call to `update()` has chenged the address list in some way or
removed a utxo from the list to prevent the user from trying to use
a utxo that doesn't exist anymore.
Fixes#10464
f321x
423cc678c8