PlasmoidHeading is basically recreating ToolBar's API, so I think there's some merit in using that instead of PlasmoidHeading

This would basically involve styling ToolBar like PlasmoidHeading and doing a mass replace everywhere

The main benefits are:

1. One less plasma specific component. We're using semantically appropriate components
2. Migration should be relatively straightforward i.e. PlasmoidHeading -> ToolBar, PlasmoidHeading.Location ->ToolBar.position
3. There shouldn't be any problem with back compat, since we don't use PC3 version of ToolBar *anywhere*.

There are few downsides as well:

1. It's kind of weird having system tray mess with PC3 components, especially when they should have identical behavior to their QQC2 counterpart
2. Negative inset/rounded corners. Kind of a deal breaker for in-content toolbar, although I'm not sure if that's a thing. Probably won't be an issue in KF6(?)