Add invokables to add/remove mapItemViews

This patch adds two new methods to QDeclarativeGeoMap, to allow users
to add or remove MapItemViews at runtime.

[ChangeLog][QtLocation][Map] Added methods to add or remove MapItemViews
at runtime.

Task-number: QTBUG-55782
Change-Id: I4e612a9476a864331f61d49dac811a8069ae010f
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>