Switch Geeqie from C to C++

[xsdg]

This has been discussed extensively on the mailing list, and after conversation between @caclark, @mowgli, and myself, it seems like this is the direction that the project is headed.

Related email threads:

• https://www.freelists.org/post/geeqie/The-future-of-Geeqie,6
• https://www.freelists.org/post/geeqie/gqpp-C-geeqie-experiment
• https://www.freelists.org/post/geeqie/Please-trial-C-geeqie

At this point, the prototypes I've built compile in both GCC (with -fpermissive, but no code changes) and clang (with significant code changes). The staging for the project transition will go as follows:

1. Change from mix of hyphens and underscores in source filenames to only hyphens (see also #1006 )
2. Rename files from *.c to *.cc. Also, add the changes to meson.build to get things compiling with GCC. This will temporarily break clang compilation
3. Add the code changes required to get things compiling with clang as well. Because there are thousands of code changes (mostly adding typecasts), this is a lot riskier than step 2, which is why it makes sense to perform it as a separate step.