Naming convention

[gucio321]

from README.md:

• For functions, 'Im/ImGui/ig' is trimmed.
• If function comes from imgui_internal.h, Internal prefix is added.
• Struct fields (if any exported) are prefixed with word Field

Also, Get prefixes are removed

Because we already generate most structs with their fields. I suggest to edit this convention:

• return Get prefixes for struct getters
• remove Field prefix for struct fields

@AllenDang what do you think?

[gucio321]

Also, we will be able to remove some extra_types (Vec2/4, Color)