wifi: Fix bug in Known Wi-Fi Networks dialog

The "Forget" button would only update it's sensitivity after the first select and deselect, when selecting and deselecting rows in the "Known Wi-Fi Networks" dialog. When selecting the first row, it would go from disabled to enabled. Then deselecting that row would cause the button to go from enabled to disabled. Selecting any rows after that would no longer update the sensitivity and make the dialog essentially useless.

The issue was, that the signals "add" and "remove" where being expected to be emitted when the connection list updates its rows. However, neither CcWifiConnectionList nor GtkListBox emit these signals. The fix was, to emit these two signals at the appropriate locations. The signals have also been renamed to "add-row" and "remove-row" to make their purpose more clear.

Closes #1824 (closed)