Clicking buttons during run/simulate mode

New Display Option "Enable Mouse Clicks While Running" enables buttons
to be selected while the simulation is running (not paused).

This works as follows:

A click on a button is processed whenever Windows gets an opportunity
to process an event (Planimate yields to it). The button will visibly
react to the click but the click message will be posted to the model
FEC to be processed as the immediate next event.

The result of this is:

• models which are routine intensive (eg: searching) will be rather
  unresponsive to the mouse whereas models with animation will be OK
• models which have zero time logic without ANY capacity or intensive
  routines will take a while to get round to responding to the mouse click
• The button clicks can occur and be processed before the completion of
  side effect actions as a result of the current operation.

This last point is particularly important. I may have to disable buttons
which can influence a model's run (eg: broadcast) or handle them specially
but this has not been implemented at this stage.

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idkbase note 29