Misc bug fixes

- fixed a bug where the dialogs would disappear in Qt version before you could
    use them
- added a fix for supporting a change to the astropy API for coordinates
    having an attribute name change from 'degrees' to 'degree'
- added a fix for an error message in the Qt version in the Zoom plugin for
    uninitialized window dimensions
Merge remote-tracking branch 'origin/master'
Add support for alternative WCS packages

Other WCS packages besides astropy.wcs can now be used with Ginga:
astlib and kapteyn are also supported.
Merge branch 'master' of github.com:ejeschke/ginga