Simplify/automate module enabling/disabling

The modules are now controlled directly from `karuibar.c`, and the
module headers will be included with two different macro definition
states, allowing to reduce the boilerplate for each module down to 5
lines.

`config.h` is no longer necessary and is thus removed.

Makefile targets are added that do the symlinking and module-including
(`modules.h`).

Enabled modules now reside in `modules` and symlink folders to
`modules.all`, that contains all modules contained in karuibar.