there should be no minimum size for visual rendering window

[jonorthwash]

Currently the visual view can only be resized as small as the initial bitmap rendering of the graphviz output. This is also the size it's initialised to.

This is not ideal because for large trees and/or small displays, the tree may not fit on the screen at that resolution and the user should be able to shrink it.

Ideal behaviour:

1. The view window should initially be sized no larger than the display area. Given proportions of screens and typical trees, the restriction will usually be on height, but width should also be checked
2. The view window should be resizable down to some minimum. Probably not 0x0, but maybe the size of a single glyph, whatever that might be. Instead of calculating this, hard-coding 10x10 might be sane enough.

Additional behaviour that might help would be zooming and panning options in the view window that affect the image view but not the window size. This can be addressed in a new issue.