wifi-manager: Fix leak of Wi-Fi connection
This MR cleans up a few logical bugs around Wi-Fi connection activation.
- Fix leaking of Wi-Fi connection in activation functions.
- Use
g_signal_connect_object
to ensure the network is alive when the callback is called. - Disconnect the network from signal once the Wi-Fi connection crosses "activating" state.
- Remove unused headers.
Closes: #1099 (closed)