viewmanager: allow creating more than 1 instance of a view

Previously, managed views were always stored in a dictionary using non-unique keys. This commit adds a method to the ViewManager component allowing for automatic unique key generation if the base key (QML component path) is the same. This is used for dialogs because we need to possibly open nested modal dialogs.

Gitlab: #1367
Change-Id: I94e0ef0e0ae79689445be0409dd902dc74554e43