useractionmodel: Make execute invokable.

Also change a proxy model property to a model interface. The
bindings have trouble exporting the proxies. This is probably a
Qt bug. This has no side effect I am aware of. They both share
the same interface.

Also remove commented out code which inits the role names.

[SS: add change missing from commit msg]

Change-Id: Ib3d2e3ed3b96d9d83a711a9005235d886840dfe5
Signed-off-by: Stepan Salenikovich <stepan.salenikovich@savoirfairelinux.com>
Reviewed-by: Stepan Salenikovich <stepan.salenikovich@savoirfairelinux.com>