Add asynchronous call with default value function

Makes it possible to easily get a value asynchronously while
having a default value assigned right away.
Once the asynchronous call returns, the default value can be
replaced by the actual result.
Make sure the pdb module is imported when starting pdb shell

Turns out I've managed to remove the import when cleaning up the Python startup code. :)
Still, better to load it when needed and not always.
Also cleanup the pdb startup code a bit.
Move the debug button to upper right

As not to "collide" with the zoom buttons.