Design: Consider elimating the initial data argument from the display panes

[seantrons]

Even though the mandatory specification of both data and the callback has been removed in favor of allowing either to exist standalone, users could just write dummy callbacks for static viewers. Additionally, for the cases where they want a set of default arguments to be applied on the first render, such arguments could be specified as the initial values for the control widgets.

In just about all ways, this would simplify the existing interface.

• Remove the need to specify keyword args to distinguish between data and callback in display contructors.
• Minimize the likelihood of users specifying duplicate initialization parameters like below:

ip_gt = ImagePane(callback=make_gauss_template)
ip_gtm = ImagePane(data=callback_tm_gauss(128), callback_tm_gauss)

[seantrons]

Changes enacted in: https://github.com/seantronsen/qtviewer/commit/d865b06ecb8e1fe81dd87436d8ef8746de9c66fb

closing.