This time with more manual checking and using git blame -M -C, so that
a few cases of copied code get a copyright notice corresponding to
their initial introduction.
Both classes did the same thing at the same time, with the only difference
that C4GUI::Resource was vaguely more associated with GUI stuff. Some
time ago, C4GUI::Resource could be freed during the game, but not anymore.
This saves some lines of code and one redundant class, but shouldn't change
anything besides progress bar being textured a little earlier during
startup.