plugins/overview: Load screen delegate asynchronously

The compilation step still takes a significant amount of time on the
first launch. On my machine, it's around 1s. Even if it happens just
once, freezing the session for 1 second is not great.

This change makes the overview effect load main.qml asynchronously
when plasma session starts. By the time the session is loaded, it should
be ready.

CCBUG: 455780