SearchToolbar: Modernise, prepare for Gtk4

[jeremypw]

View hiding whitespace change.

Any other Gtk4 prep needed?

Keeping as draft until pending release goes out

[jeremypw]

@danirabbit Can we clarify whether the license should be LGPL-3.0-only or LGPL-3.0-or-later? According to https://gnome.pages.gitlab.gnome.org/vte/gtk3/index.html the Gtk3 Vte library is the latter and https://gnome.pages.gitlab.gnome.org/vte/gtk4/index.html indicates the same for the Gtk4 library.

[danirabbit]

@jeremypw it looks like this file was originally licensed only, but as a legal representative of elementary I approve relicensing as LGPL-3.0-or-later :)

[jeremypw]

@danirabbit So does it need two license statements? One LGPL-3.0-or-later from 2025 and another for when it was LGPL-3.0-only?

[danirabbit]

@jeremypw no we can just re-license it to LGPL-3.0-or-later since elementary holds copyright here