Implementing elements UI as the UI library for DPF

[SeleDreams]

I recently ended up discovering the "Elements" UI library, it seems to be a C++ UI library under a non viral license that was from the beginning developped to work well with audio plugin frameworks to compete with JUCE
I feel like this could be a very good thing to integrate it to DPF to make dpf easier to work with.

I am going to personally look into implementing support for it, I think I saw at one point that it had a pugl version somewhere, which should make its implementation easy

[SeleDreams]

https://github.com/cycfi/elements this is elements btw

and seems like this is the PR which implemented pugl a while back but hasn't been officially added yet cycfi/elements#111

I probably could use it as reference with the latest version of the code

[falkTX]

same as per #355 use https://github.com/DISTRHO/DPF-Widgets that is where such thing belongs.
For DPF I have interest on pushing the event logic around widget handling, but besides nanovg there wont be another official graphics API in this repo