Since the introduction of multi-select, we have had both `on_selection`
and `on_selection_change`, the latter of which was only invoked when a
change in selection came in through the model.
This removes `AbstractView::on_selection` and replaces it usage with
the more explicit `on_selection_change` everywhere.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
This enables the user to view and navigate classes with a TreeView that
is updated by the LanguageServer as it parses the code.
It offers a new neat way to view the project's structure :^)
