Improve warnings & info to the user

[jluethi]

Show it in the napari interface even when it's run from a notebook.

The show_info messages are just disappearing in a void if napari is run in a notebook => currently using a print & show_info for those, so they show up in the console & jupyter lab at least.

Warnings: are shown in console & notebook, but not in the interface when running from a notebook.

Also track this, switch to show_warning once it's available to make sure my warnings are still visible: napari/napari#3787