Feature - DPF: plugin preset handling

[dromer]

While DPF supports "programs", which are built in presets, it would be nice to have a proper preset system where users can store and import preset files. A basic json format could be configured for this.

This would require:

• include minimal json library (cjson?, json?)
• cross plat default storage location
• cross plat integration with file/location browsers
• basic integration in ImGui generator