As mentioned in https://github.com/microsoft/terminal/issues/9354#issuecomment-790034728
`GETSET_SETTING` is too visually similar to `GETSET_PROPERTY`, but with a _VERY_ different meaning. I think that merely changing the name of the macro would make it harder for us to make this mistake again.
First step towards #8415:
* Introduce `PaletteItem` and derive from it to provide native support
for tabs and command lines (`ActionPaletteItem` / `TabPaletteItem`,
`CommandLinePaltteItem`)
* Remove business logic behind PaletteItem from palette (aka dispatch
commands and preview tabs externally)