frontend: editor: make widget placement and deletion consistent

[ES-Alexander]

Current behaviour

• Regular widgets are added by dragging the card into the main View, then releasing the card to spawn the widget in the centre, after which it can be repositioned
  • They can be reordered (in the layer stack) by dragging them in the "Widgets in View" list
  • Deletion is only possible via the "Widgets in View" list
• Mini-widgets and input-widgets are dragged directly into their containers/bars
  • They cannot be reordered or moved via the sidebar
  • Deletion involves either:
    1. dragging into a trash bin that covers part of the View area (which can cover areas you actually want to place them), or
    2. pressing the trash icon in the "Widgets in View" list in the left sidebar, for those that are included

Expected or desired behaviour

1. Widget placement should involve dragging the actual widget where you want it to go
2. Widget reordering (as that applies to different widget types) should be possible by dragging cards in the "Widgets in View" list
3. Widget deletion should be possible by either:
   1. dragging the widget into the bottom bar, which should turn into a trash bin / "Remove" region while any widget is being dragged (including during creation, so you can change your mind easily), or
   2. pressing a delete button in the "Widgets in View" list

Prerequisites

• I have checked to make sure that a similar request has not already been filed or fixed.