Add numbered viewer variables to be able to access multiple created viewers

[dalthviz]

Closes #40

Enable to access all the available viewers by adding a viewer{number} variables as needed. For example, if you create from a single script to viewers you will be able to access/interact with both: one via a viewer0 variable and the other one via viewer1. A preview:

Notes

• One thing that this implementation is not covering is to remove the variable referencing the viewer once the viewer is closed/deleted. So, with this implementation, you will be able to access the deleted viewer from another viewer/napari mainwindow instance and then getting an error when trying to do any operation with the closed/deleted viewer variable. Tried setting some clean up logic at the console closeEvent using either self.push (setting the respective numbered viewer variable to be something like None) or calling the shell reset_selective and drop_by_id methods but none of those calls helped as far as I was able to check 🤔 Any ideas to handle this are appreciated!

[Czaki]

Would it be possible to save the current viewer number and remove the reference in closeEvent?