Refactor `DmsBox` widget and improve the Add Catalog Object form

This MR introduces a bunch of changes:

1. dms literals (based on the C++11 user-defined literals feature) [Needs testing / review]. For example, 1.0_deg produces a dms constant with a value of 1.0°, 1.0_h produces a dms constant with a value of 15.0°, 1.0_rad produces a dms constant with a value of 180/pi°, "12:15:22"_hms produces a dms constant with a value of 12h15m22s, "-87:22:11"_dms produces a dms constant with a value of -87°22'11" etc.

2. Major refactoring of DmsBox to make for a smoother API that prevents accidental introduction of bugs in unit conversion, as well as simplifying the code using QLineEdit::setPlaceholderText (which presumably did not exist when the code was written and hence needed a workaround).

3. Improvements to the Add Catalog Object UI, including widening it, using some placeholder text to guide the user, and most notably, using DmsBox widget for RA and Dec entry.