Make keyboard events working for GQLView

Hi,
When working on my Notebook, I don't have a scroll-wheel mouse, so I was missing keyboard interactions to zoom.

Looking at the code I found, that + and - should actually work but didn't in practice, because the view never got the focus. So I changed that the focus is acquired when someone clicks into the view.

On my keyboard (as on a typical American keyboard), the - and + key are next to each other, but the '+' requires to press shift - the 'non-shifted' character is just an '=' sign. So to zoom in, this change allows to react on both, + and = sign for zooming (as is done in many other graphical applications).

Finally, while at it, I added a 'c(enter)' keypress; often it is simple to get lost in panning, so with 'c', things are nicely re-aligned in the middle.

Thoughts for follow-up keyboard addition: maybe have cursor-keys for simple yaw and pitch operation (essentially rotating through the horizontal or vertical axis as seen on the screen)

thanks for the awesome OpenScad ... I just got a 3D printer for testing, and for me as a programmer, I felt right at home in the language.

-h