Public function for converting hex strings to RGB Longs
pdVisualThemes: start implementing central color handling

I won't actually be tackling full-on theme support for 6.4, but for the
various custom UCs getting added, I'd like to tie them into the theming
engine now (as opposed to hard-coding all their colors uniquely).

The plan at present is to have each UC manage its own theme
interactions, rather than having a central function apply them via
properties.  Besides simplifying the design of each UC's property list,
this will also allow for better optimization, as individual UCs know
exactly what needs to be redrawn when the program theme changes.
(Similarly, certain translation engine elements will be moved out of the
central "translate everything" function and into individual UCs as
well.)  These UCs will all expose some sort of "update against current
theme" function, so that when the program theme changes, they can all be
updated automatically.

To that end, I'm setting up some central theme engine components prior
to 6.4's release.  That way, when themes are implemented in 6.6
(hopefully), all UCs will already be tied into the central theme engine,
making testing and implementation a breeze.