Cleanup/rework python plugin implementation

[eidheim]

[eidheim]

Note: check out https://github.com/wjakob/pybind11

[zalox]

I have prioritized the dialogs. Making sure the experience is equal between platforms. I have time to continue on Thursday, but maybe earlier. After that i'd like to work on this.

[eidheim]

Sounds good!

[eidheim]

Be aware that I still need to do some work in source* before once can implement for instance python autocomplete through plugins, I probably do not have time to do this before December, but I know roughly how it can be done. Same with diagnostics. Refactoring however, accessible through anonymous functions, should be doable with only a couple lines of code. Safest for the time being is to work on snippets. However, try not to modify the existing source too much for the time being. I think python plugins can be implemented without significant modification of the existing source, but not 100% sure.