Group items on a folio so they move and copy together

[ispyisail]

Items on a folio cannot be grouped. A symbol with a frame drawn around it, a
picture with its caption, or a set of symbols with their notes all have to be
selected piece by piece every time they are moved or copied. This has come up
in #349 and on the forum (#2467,
#356).

Proposed:

• Group and Ungroup in the Edit menu and the right-click menu of a
  selection.
• Clicking any item of a group selects the whole group. Moving, the arrow
  keys, copy, cut, delete and rotate then work on the whole group.
• Clicking again on an item of a selected group selects just that item, to
  edit it on its own.
• Pasting a group makes a new, separate group.
• Wires are never part of a group; they follow their symbols as they do
  today.

Existing files: a grouped item gets one extra attribute in the project
file. Projects without groups save exactly as before. Older versions open
grouped projects and ignore the grouping.

Not included: nested groups, and composite devices with their own terminals
(#617), which would build on this. Saving a group for reuse already exists:
select it and use "Créer un template".

Questions:

• Is this wanted?
• Is "click again to pick one item inside the group" the right way to edit a
  single member?
• Should a group have a name, shown when it is selected?