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 *
They were previously stored inside the Locator widget. I moved them to
the globally visible ProjectDeclarations class so they can also be used
by other widgets.
The Locator now keeps a cache of the declared symbol in a document.
The language client updates that cache whenever it gets an update from
the language server about declared symbols.
This allows searching for symbol declarations in the Locator, in
addition to file names.
Closes#5478
