This patch adds a visibility state to GUI::Action. All actions default
to being visible. When invisible, they do not show up in toolbars on
menus (and importantly, they don't occupy any space).
This can be used to hide/show context-sensitive actions dynamically
without rebuilding menus and toolbars.
Thanks to Tim Slater for assuming that action visibility was a thing,
which gave me a reason to implement it! :^)
Briefly flash the menubar menu containing the keyboard shortcut action
to give the user immediate visual feedback on their interaction with the
system.
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 *
