Some UI fixes and improvements

RoomSettings: Make room avatar bigger

This is literally the page about the room, we are not short on space here. Make it shine.

RoomSettings: Port disabled text fields to readOnly

Improves visuals, and allows to select & copy text. Also hides Save button when there's no chance it would get enabled.

Depends on very recently merged MR in kirigami-addons: libraries/kirigami-addons!153 (merged)

ChatBox: Fix jittery ChatBar

ChatBar animated its implicit height, and there is no way for us to force it animating on integer values only. It is contained in a Layout type, and QtQuick.Layouts are known to round up size of their items and position them on an integer scale. All in all, is caused bottom of chat bar (action icons row and the whole bottom edge) to flicker during height animation.

Give "+" tool button a proper a11y role and an RTL-friendly modal menu