Differentiate singleton tools from multi-instance ones ?

[NicolasAnquetil]

We just made Model Browser a singleton.
This is because when loading a model it is loaded "In Moose" and not just in the browser.

• the Moose Overview browser should clearly be a singleton
• The Tag browser could be a singleton too, tags are for all Moose
• the Logger seems like another good candidate for singleton too
• And the question was raised for Moose Critic in Moose Critic: Imported rules are not saved in the Moose Critic Browser when closing and reopen it #1121

Other tools are clearly multi-instance ones:

• All the visualizations
• the query browser

The questions are:

• do we agree on this separation ?
• should we make it more clear ?
• if so, how ? (separate groups in the Moose menu? Different graphical hints, like a special title or icon for the singletons?)