Member Page

Adds a member page to the room details window. The page shows information of all members in the room, including mxid and powerlevel. To filter room members it provides a search bar.

I wrote a three-dot menu for each member to kick them and change the power level, but for the moment I did not include it because it causes an unbearable O(n²) slowdown in UI rendering. I contacted the GTK folks, but they did not immediately had a solution, so this requires further investigation.