Reorganize EditorView visual layout
Main changes:
- There's now a Kirigami Separator between the editor and the preview
- While horizontal, the editor scrollBar is now on the left
- The background color is now from the 'Window' colorSet and not the 'View' one. This meens that less different color are used which make the UI looks less cluttered.
Other changes:
- The GridLayout no longer uses columns/rows value change to pass from horizontal to vertical, use flow instead.
- The editor is now focused by default when a new note is loaded and its cursor is placed at the end of the note