gdb: introduce rgb_color type to simplify existing code

This patch replaces the raw uint8[3] buffer used to represent RGB values
with a more convenient wrapper, rgb_color, around std::array<uint8_t, 3>.
It also changes the return type of ui_file_style::color::get_rgb to
rgb_color instead of filling a caller-provided buffer, and updates all
callers accordingly.

This expected benefit of this change consists in:
- removing the manual size handling.
- proving accessors without using hard-coded indexes.
- making the API safer.
- simplifying call sites.

This refactoring does not introduce any functional change.

Approved-By: Tom Tromey <tom@tromey.com>