Fix crash when pressing ESC after external tools plugin was un/reloaded

The new conntect() syntax along with a lambda is not automatically
destroyed when the plugin is unloaded, since the connection does not
track the "this" pointer that is captured in the lambda function,
a typicall programming error :-/

This change turns the connection into a proper member function, so
the connection is cleaned up when the plugin view is deleted.

FIXED-IN: 19.12.1