Display ASDF header when using ds9

[perrygreenfield]

This provides a way of displaying header information for images displayed in ds9.

This functionality is provided on the ASDF dialog side with a button the main dialog window. The corresponding button will be greyed out if no image has been selected, but will become active if an image is currently seleected.

The header is displayed on the Python side in a child window. It is possible to keep multiple child header displays open as each has a quit button (e.g., one can display one image from a specific file, open a header display for the file, and then select a new file and display a new header window while the previous one is still open.

This uses the AsdfFile info() method with large limits on rows and columns (the opened window is scrollable). It was necessary to strip the bolding and italic markup from the method's output since from what I could tell, doing the same for Tkinter appeared cumbersome and perhaps slow. To make Roman displays less annoying (and for all ASDF displays for that matter), the asdf_library and history sections are omitted (a future enhancement could allow these to be optionally shown). In addition, in the case of Roman, the roman.meta.cal_log section is also suppressed.

Closes Issue #13

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