Have TextDocument listen for state changes on the internal undo stack,
and forward those to all clients via a new virtual function.
This simplifies updating the can_undo / can_redo states of TextEditor.
Until now, this has been hackishly tracked by the TextEditor app's
main widget. Let's do it in GUI::TextDocument instead, so that anyone
who uses this class can know whether it's modified or not.
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 is a little bit messy but the basic idea is:
Syntax::Highlighter now has a Syntax::HighlighterClient to talk to the
outside world. It mostly communicates in LibGUI primitives that are
available in headers, so inlineable.
GUI::TextEditor inherits from Syntax::HighlighterClient.
This let us to move GUI::JSSyntaxHighlighter to JS::SyntaxHighlighter
and remove LibGUI's dependency on LibJS.
