Use mouse button release rather than press to activate a hotspot

As described in issue #412, when using extended mouse coordinate (SGR) mode, the mouse button release sequence would be received by the BBS after the hotspot-activated menu option was executed and then that button release sequence possibly passed through to the a child script or program that didn't know how to handle or ignore it.

To resolve this, the BBS now ignores ("eats") the mouse button *press* sequence and only acts upon the mouse button *release* sequence. This is more aligned with how button-clicks work in mouse-control/GUI applications anyway.