[Studio] Progress entry editors

[Stefan Seelmann <se...@apache.org>]

Hi all,

Pierre-Arnaud and I started to implement the modifications of the entry
editors.

Currently we have two entry editors
- The single-tab editor that behaves like the current entry editor
(DIRSTUDIO-517)
- The multi-tab editor the opens one editor for each entry. It follows
the open-save-close lifecycle (DIRSTUDIO-518).

The default editor could be selected from the preference page. Otherwise
another editor could be selected from the browser's context menu.

The open mode is also implemented, it is possible to select if the entry
should be opened using single or double click.

My next steps are:
- Add the LDIF entry editor (DIRSTUDIO-519)
- Make the modification mode configurable (immediate commit vs.
open-save-close) (DIRSTUDIO-518)

Then we should change the way how modifications are sent to the server
(DIRSTUDIO-513). I also encountered some issues with concurrent
modifications when multipe editor are opened. I'll start a discussion
about that later.

Kind Regards,
Stefan