fix clearing project name on opening project properties page.

The problem exists only on Windows with wxPython-3.0.

This happens because callback for EVT_KILL_FOCUS for a first text
control (project name) is called just after creating page with project
properties. It happens before initializing text controls from PLCController.

fix problem with debugging external string variables

Debug update stopped if external string variable added to debug
variable panel.

fix problem with clearing non-empty location field after changing type of config variable

introduced by cd9db17 ("clear variable location cell in variable
panel, if variable class or type changed").

Closes #26