This commit also introduces a tempprary fix to an issue where fab is
displayed on wrong screen when switching between bottom tabs rapidly.

call setButtons with empty fab object to remove it

This is needed since some of the data we need to send in the events
can not be serialised and added to a Bundle.

When Snackbar was displayed while another Snackbar was displayed,
the last ref to the current Snackbar was removed and it didn't get removed
when onScreenChange was called.

* Created Snakbar class to encapsulate Snackbar logic.
* Clicking the action now emits event to current screen
* Action color works