[Qt-creator] Multiple IEditors for one IDocument

[Jochen Becher]

Hello,

for my ModelEditor plugin I would like to use many IEditor (one for each
diagram) associated to one IDocument (representing the XML model file).
Currently I must create a new document for each diagram editor because
of limitations in the design. For example displayName is only defined in
IDocument, not in IEditor, and different editors should show different
names (in my case the diagram's name).

Having many documents related to one physical file has some annoying
consequences. If you modified the model you will be asked if you want to
save the file for each closing diagram even if you do not close the last
diagram (and thus the document). If you close all editors you will be
asked to save all diagrams instead of only the single file. And more...

Before I start implementing the changes I would like to discuss it:
* add getter IEditor::displayName() (default implementation falls back
to IDocument), IEditor::uniqueName() and some setters.
* change combobox in toolbar showing all open editors (instead of open
documents)
* introduce new side bar window "Open Editors"
* extend Core::EditorManager to allow opening another Editor for an
existing editor (with a parameter that tells the editor to show which
view on the document).

There are a view more changes I have not yet though about: the draggable
icon in the toolbar (which stores a file path but no identifier for the
editor) and the state of open documents (instead of open editors) which
is saved on closing QtCreator.

Regards, Jochen