main-window/contacts-pane/contacts-list: fix header buttons on row checkbox toggle
As described in #198 (closed), toggling the row checkbox currently shows the favourite and edit contact header buttons even if no contact is displayed.
Added last_row_checkbox_toggled signals in contact-list and contact-pane, and handled updating the UiState after a toggle in main-window, similarly to how the existing contacts_marked signals work. This stops the row checkbox from making header buttons appear if no contact is currently displayed, as expected.
Fixes #198 (closed)