pdInput: implement custom Click event

Not gonna lie, this little feature makes me oddly happy.  One of my
recurring issues with complex custom-drawn UI elements like the layer
box is that the user can click on > 50 unique elements at any given
time, each of which does something different.

Because VB6 is stupid, it doesn't report coordinates or buttons on click
events, which forces me to use MouseDown/Up instead.  But these are the
wrong tool for the job, because if the user wants to do something like
click-and-drag the list to scroll it, accidentally starting their drag
on a clickable element causes a "click" to trigger, which is totally
inappropriate in an age where touchscreens have taught us that
scrollable lists don't work this way.  Instead, click-drags should be
automatically separated from clicks (which don't involve a "movement"
step, just a quick down/up of the mouse).

I am extremely tired of implementing those kinds of checks manually, but
now, I can simply use a Click function with the coordinates and buttons
all reported right there in the event parameters.

As with other mouse events, this new event will also report X-button
clicks.