Introduce .spx/.gmx as Go+ class files

[xushiwei]

Let's see an example.

Here is content of Foo.gmx:

var (
    Bar Bar
)

func onStart() {
    Bar.clone()
}

Here is content of Bar.spx:

var (
    level int
)

func onStart() {
    level = 1
}

[xushiwei]

import spx "github.com/goplus/spx"

type Foo struct {
    spx.Game
    Bar Bar
}

func (p *Foo) OnStart() {
    p.Bar.Clone()
}

type Bar struct {
    spx.Sprite
    *Foo
    level int
}

func (p *Bar) OnStart() {
    p.level = 1
}

[xushiwei]

https://github.com/goplus/gop/releases/tag/v1.0.0-beta5