Editor and labels inconsistencies #582

Several bugs in the editor and the label pages.

• Deleted notes are editable (regression of v2.1.2): readOnly = !currentNote.deleted && … makes the editor editable precisely for the notes in the bin.
• An edited archived note appears in the main list: the editor and the title always save through notesProvider(status: available), which adds the archived note to its state until the next reload.
• Label filter reset when switching labels: currentLabelFilter is global. With goNamed, the page of label B is created before the page of label A is disposed, and A's dispose sets the filter back to null. On label B, edits, new notes and the search then target all the notes.
• A note created on a label page doesn't get that label: it shows in the label's list, then disappears at the next reload.
• Keyboard closing when creating a note with several note types (probably Note focus lost on opening when multiple notes types are available  #571): the expandable FAB is closed after the editor is pushed. closeAddNoteFabIfOpen also ignores the FAB of the label pages.
• Undo / redo not updating after a formatting action (Undo/redo buttons don't update appropriately  #565): fleather pushes to its history with a 500 ms throttle, but canUndo / canRedo are only read right when the document changes.
• Smaller ones:
  • "Next" on the title keyboard doesn't move to the content (onSubmitted: (_) => widget.onSubmitted never calls it).
  • The title creates a new TextEditingController on each build, and the editors' controllers are never disposed.
  • setDeleted toggles note.deleted instead of setting it.
  • Undoing the deletion of several archived notes uses a widget ref that may be disposed, and shows a second snack bar.
  • A malformed Markdown link throws a FormatException.
  • setArchived and toggleLock remove the empty notes from the list they receive (the selection).