Improve the "Create a Room/Space" and "Select Existing Room" dialog

First of all, these are now all separate dialogs instead of shoving all of these functions (which are only marginally related) into one single dialog. We also convert these to in-app Kirigami Dialogs, which look a bit nicer. I also touched up the UX in some places, such as adding descriptions which were previously available to translators. I also hid some not oft used options like setting a topic, which almost nobody does before creating a room/space.

Create Room

Before	After

Create Space

Before	After

Select Existing Room

Before	After