FlatCAMDraw is responding to key press when inactive.

[Juan Pablo Caram]

The following error was thrown when hitting a key on the keyboard while having focus on the plot just after starting the program:

[DEBUG][Dummy-1] FlatCAM is up to date!
[DEBUG][MainThread] button=1, x=137, y=199, xdata=0.042453, ydata=0.328302
[DEBUG][MainThread] No active tool to respond to click!
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/matplotlib/backends/backend_qt5.py", line 315, in keyPressEvent
    FigureCanvasBase.key_press_event(self, key)
  File "/usr/local/lib/python2.7/dist-packages/matplotlib/backend_bases.py", line 1816, in key_press_event
    self.callbacks.process(s, event)
  File "/usr/local/lib/python2.7/dist-packages/matplotlib/cbook.py", line 540, in process
    proxy(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/matplotlib/cbook.py", line 415, in __call__
    return mtd(*args, **kwargs)
  File "/home/jpcaram/flatcam/FlatCAMDraw.py", line 937, in on_canvas_key
    response = self.active_tool.on_key(event.key)
AttributeError: 'NoneType' object has no attribute 'on_key'

Action items:

• Catch the error or prevent it when no tool is active.
• Or, make sure no FlatCAMDraw instance is around when not "active".

[Juan Pablo Caram]

This was causing issue #93 by preventing the code to reach the other event handlers when the exception is raised.

The exception has been prevented in the code.

[Juan Pablo Caram]

There is in fact a unique instance of FlatCAMDraw. Added activation/deactivation code taking care of the signals and callbacks. Commit 8da39d5.