Addon API should have a way to know if the engine will pause

[dvojtise]

Currently, addons can register on step events.
However, similarly to Sirius view, (#122) some Addon may not be interested to be triggered too often.

In order to implement similar strategies as suggested in #122, it would be useful that the Addon API provide some means to access the information about whether the engine step is about to pause (due to breakpoint, a manual pause or reaching the end of a step over, step return, step into)

With this information, the addons could provide smarter behaviour.