Application loader support

[bigtimebuddy]

Adds convenience support for loader to the Application class.

const app = new PIXI.Application();

// Pause rendering
app.stop(); 

// Load some resources
app.loader
    .add('logo', 'images/logo.png');
    .load(() => {
       app.start(); // resume rendering
    });

Added

• Adds loader property the Application class, e.g., app.loader.load()
• Adds constructor argument options.sharedLoader to Application to opt-in to using the shared Loader.
• Adds PIXI.loaders.shared as an alias for PIXI.loader (conforms to Ticker usage PIXI.ticker.shared).

Fixed

• Fixes issue when excluding the loaders module npm run dist — -e loaders (deprecations were throwing issues for deprecated Resource, e.g. 'isJson`, etc).

[englercj]

I think this is fine, not crazy about the PIXI.loader -> PIXI.loaders.shared change, since this is something that is so commonly used. But I also don't feel that strongly about it.

[bigtimebuddy]

@englercj Fair point. I re-added PIXI.loader.

[lock]

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