Prevent being able to install plugins that are only available via PyPI on a bundle installation

[dalthviz]

Part of #110

Explores the option to prevent installing plugins that are only available through PyPI on bundle installations. This particular implementation uses some previously implemented logic to tag plugins as unavailable (disabling the install/action button, changing the plugin item style and showing a warning icon + tooltip). A preview:

Note: To check locally the items being disabled and the dialog layout as seen from a bundle installation, you need to replace/change

napari-plugin-manager/napari_plugin_manager/qt_plugin_dialog.py

Lines 72 to 73 in dbcd362

	ON_BUNDLE = running_as_constructor_app()
	IS_NAPARI_CONDA_INSTALLED = is_conda_package('napari')

for something like:

ON_BUNDLE = True
IS_NAPARI_CONDA_INSTALLED = True

Also, if this option gets favored over PR #111 probably some further styling needs to be done to the 'disabled' plugin items (to properly aling the warning icon, disable the installation info widget, etc)

[psobolewskiPhD]

If this goes in, could the text-entry box at the bottom still be used to force a pip install?
I think I lean to the other option of letting a user force pip if they want....

[jaimergp]

Closing this in favor of #111. We can still revisit in the future if we need something stricter. Thanks @dalthviz.