codegen: Add presets json support

[gucio321]

What is this?
currently, gengo.go (and randomly other files) have various maps with hardcoded strings.
The purpose of this change is, to move all of them and store as a single object.
I've decided to split this into 2 parts:

• presets.go contains a go struct. Its instance is stored inside of a Context.
• cimgu-go-preset.json contains actual values compatible with cimgui-go.

Why?

• To put everything hard-coded in 1 place
• To make our codegen independent on hardcoded values (maybe someone will use it somewhere else?)

There is a new codegen -preset flag to set path for this json file.