Makes scrolling position persistent, and takes some care to
1. delay restoring until the text is laid out,
2. throttle storage a little, so that we don't use too much I/O.

Component.onDestruction correctly saves the position on exit. It can be
tested via D-Bus by calling method quit() at:

- Session bus: org.kde.plasmashell
- Object path: /MainApplication
- Interface: org.qtproject.Qt.QCoreApplication

Cursor position restoration can be tested by assigning a global shortcut
to an applet, so that it can become focused without actually clicking
on it with a mouse and messing with cursor position. Text selection is
not handled, and I don't think we should even bother with it.

BUG: 449150