Add missing spawn, position, force_screen and max_clients signals.

Without these the following will hapen:
```
W: awesome: luaA_object_emit_signal:286: Trying to emit unknown signal 'property::position'
W: awesome: signal_object_emit:239: Trying to emit unknown signal 'property::position'
W: awesome: luaA_object_emit_signal:286: Trying to emit unknown signal 'property::force_screen'
W: awesome: signal_object_emit:239: Trying to emit unknown signal 'property::force_screen'
W: awesome: luaA_object_emit_signal:286: Trying to emit unknown signal 'property::position'
W: awesome: signal_object_emit:239: Trying to emit unknown signal 'property::position'
W: awesome: luaA_object_emit_signal:286: Trying to emit unknown signal 'property::max_clients'
W: awesome: signal_object_emit:239: Trying to emit unknown signal 'property::max_clients'
```