Room Directory UI for exploring and joining public rooms. V1: simplistic server + network facilities

Reorganise src/RegisterPage.cpp

fixes #562

Requires flatpak 1.11.1

Sticker editor

Open profile when clicking avatar

So IDNs are not rejected. Invalid server names will be caught later.

We explicitly set a parent. We can't assign to
ApplicationWindow.transientParent though, only to
Window.transientParent, so we just call setTransientParent in C++.

QML all the things, part 3: Raw message dialog

fixes #636