frontend: editor: simple/mobile-friendly interface reimagining

[ES-Alexander]

Context

Largely raised in #1540, but there have been other problems reported about unintuitiveness of the current edit mode, and the desire to have an editing interface that feels more like WYSIWYG, which incidentally could also make things more mobile friendly by using more of the screen for the widgets and the like.

Expected or desired behaviour

This is a somewhat alternative plan, and may be longer term.

The primary idea here is to simplify the interface by separating layout reorganisation, widget configuration, and widget adding functionalities, while highlighting components that are most useful to the task at hand.

Suggestions are split into sub-issues.