Fix #8313: use correct capitalization for TTO / DOS base-music (#8385)

DOS did not have cases in filenames. Different OS interpret
them as either all-lowercase or all-uppercase. So we try both.
All-uppercase is done by the obg/obm/obs files, and if opening
fails, OpenTTD will automatically retry the all-lowercase variant.

So for those who already have the files lowercase, nothing
changes. For those that install fresh from TTO, it should now
work out-of-the-box.

Codechange: rework how grf and ob[msg] are generated

For grfs, it now uses CMake scripts to do its job, and both grf
files are split into their own folder to make more clear what is
going on. Additionally, it no longer builds in-source (although the
resulting grf is copied back in the source folder).

For ob[msg] it now uses CMake scripts to generate the translation
files; the result is no longer stored in-source (but in the build
folder).

Although all files are available to create the GRFs and basesets, it
won't really work till CMake is introduced (which will happen in a
few commits from here)