ActionToolBar: Porting/refactoring

ActionToolBar: Reformat ActionsMenu

Just some standard formatting and root component id. Remove comment about something-something between Qt 5.9 and 5.10 — whatever it was it is not relevant anymore.

ActionsMenu: Port Instantiator's delegate to Component lifecycle hooks

Since we don't use index argument of objectAdded/Removed methods anyway (even though we probably should? to ensure the same order), using built-in Component.* hook is probably cheaper than defining extra custom methods for literally the same semantics.

ActionsMenu: Use type-safe check for Kirigami.Action

ActionsMenu: Use appropriate template types for properties

For property types, styled component types from QQC2 module should almost never be used. Use their Templates counterparts for greater flexibility and interoperability between different styles of literally the same API.

See also: #53