Fix signature of eventOccurred()

IntelliJ shows a warning at the locations where we override eventOccurred() from EventListener with Event? as the argument instead of Event. The interface is not nullable, so I think we can make this Event everywhere.

$ grep -rI --include "*.kt"  eventOcc briar-desktop/
briar-desktop/src/main/kotlin/org/briarproject/briar/desktop/conversation/ConversationViewModel.kt:    override fun eventOccurred(e: Event?) {                         
briar-desktop/src/main/kotlin/org/briarproject/briar/desktop/login/StartupViewModel.kt:    override fun eventOccurred(e: Event) {                                      
briar-desktop/src/main/kotlin/org/briarproject/briar/desktop/forums/ThreadedConversationViewModel.kt:    override fun eventOccurred(e: Event) {                        
briar-desktop/src/main/kotlin/org/briarproject/briar/desktop/forums/ForumViewModel.kt:    override fun eventOccurred(e: Event) {                                       
briar-desktop/src/main/kotlin/org/briarproject/briar/desktop/privategroups/ThreadedConversationViewModel.kt:    override fun eventOccurred(e: Event?) {                
briar-desktop/src/main/kotlin/org/briarproject/briar/desktop/privategroups/PrivateGroupListViewModel.kt:    override fun eventOccurred(e: Event?) {                    
briar-desktop/src/main/kotlin/org/briarproject/briar/desktop/contact/ContactsViewModel.kt:    override fun eventOccurred(e: Event?) {                                  
briar-desktop/src/main/kotlin/org/briarproject/briar/desktop/contact/ContactListViewModel.kt:    override fun eventOccurred(e: Event?) {                               
briar-desktop/src/main/kotlin/org/briarproject/briar/desktop/contact/ContactListViewModel.kt:        super.eventOccurred(e)