gtk: Add number of specialists next to city size in citydlg
Actually they do show that, when there is specialists. The formatting is just different when there's no specialists - it doesn't show the zero.
Tooltip is missing, though. For Qt-client that is coming in #44409.
Qt-client shows number of specialists as the additional info to city size (which tells total number of people). Gtk client should do the same. Currently that space is not used at all.