Replace libgconf

[onli]

On void, it seems like the old libgconf-2.4 is finally gone for good, which breaks simdock for me. Simdock should compile with something else. There is likely no need to use Gnome's storage API here in the first place, setting could be stored in an .ini or .toml instead.

[onli]

Solved already, https://github.com/onli/simdock/releases/tag/3.0 is the linked release.

Instead of using .ini or .toml directly I decided to rely on wxConfig instead. Given how stable wxWidgets has been over the last decade, I hope that this will avoid issues in the future.

This was also an opportunity to reduce the code base, the config mechanism had an unnecessary layer of abstraction, at least unnecessary when using wxConfig.