New Developer Tools menu (non-production builds only)

I debated building a standalone tool for building PD's new theme files,
but much like the Translation Editor, the number of PD-specific
dependencies make it much easier to simply build the tool into PD.

A new Developer Tools section will make it easer to work with items like
this, and it's already set up to be hidden in production builds.
Add Theme folders in /App and /Data

Like language files, there need to be two locations for themes: /App for
PD's default themes, and /Data for any user themes.  I don't know if
user theme support will happen in 6.6, but theoretically they shouldn't
be too difficult - just XML files with a few color constants!
Metadata browser: expose ExifTool's -htmldump option

I have mixed feelings about exposing this option for all users, but as a
developer, it is invaluable for tracking down tricky metadata issues.

To save myself some trouble, I've exposed it right there in the dialog
as a one-click command button option, but when this dialog gets reworked
to allow for editing metadata, I may revisit.