matrix: Show room topic in chat details

This needs https://source.puri.sm/Librem5/libcmatrix/-/merge_requests/74

Due to the fact that the dialog isn't recreated but kept around the topic isn't yet updated when it changes an the dialog is reopened. Showing details of another chat in between "fixes" this. We'll fix that in another round as it e.g. affects the room name as well.