Add Back Button function for layers

[BlueWitherer]

A simple convenient function for developers to quickly add a very basic back button onto their custom UI layers. Here's an example of a pink back button applied to this template layer.

The function is defined as such.

    void addBackButton(
        cocos2d::CCNode* to,
        cocos2d::CCObject* target,
        cocos2d::SEL_MenuHandler selector,
        BackButtonStyle style = BackButtonStyle::Green
    );

It's as simple as pie!

geode::addBackButton(this, this, menu_selector(ExampleLayer::onGoBack)); // or any class function!

geode::addBackButton(this, this, menu_selector(ExampleLayer::onGoBack), BackButtonStyle::Pink); // make it pretty!

It can get annoying for developers having to manually create this button every time especially when they're working with multiple custom layers; the intention behind this is to simply streamline that process.

originally suggested by @DumbCaveSpider

[BlueWitherer]

just learned gametoolbox has this but it just crashes when used