Improve the manual focus dialog for the SkyMap

The focus dialog in the Sky Map allows for manual centering of a field by specifying its coordinates. Most commonly, this would be the J2000 equatorial coordinates. To this end, this MR introduces the following changes:

* Makes the tab order more natural for mouse-free use of the dialog
* Makes J2000 the default epoch for the coordinate entry
* Fixes a bug in which the existing center point was sometimes altered by the dialog
* Refactors the code to make a single code path that does not rely on knowing whether the epoch is JNow or not (at the cost of some extra calculation).