Split configuration variables to a config.h file. (suckless style)

[MouadCharradi]

This would allow for a separation between the configuration part and the logic part of bemenu.

Thanks

[MouadCharradi]

I just start reading the bemenu codebase, maybe there are more configuration variables that I haven't seen yet. Maybe we could put those in the config file as well.

[Cloudef]

Mostly the defaults in common.c perhaps (for bemenu and bemenu-run executable)

[MouadCharradi]

I have move more values to config.h from bemenu.c and bemenu-run.c. I'm not sure if there are any default variables that are set in common, it seems to me that it just handles cli opts and changes behavior accordingly, but I might be wrong.

As for the warnings, I will try to find a way of getting rid of them (maybe through a flto option in the Makefile).
Let me know if I did anything wrong, or if there is anything else I should change or do, or even abandon this whole idea if it doesn't align with the project's vision.

Thanks. Mouad.