Add a "zoom to fit" button

[fsih]

Reasoning

It's currently unclear what the '=' button does in the paint editor. What it does now is zoom the paint editor to 100% zoom level. It seems from feedback that people would like a button that zooms the paint editor's white checkered area to fill as much space as possible in the tab.

Affordances:

• Quick editing of Backdrops
• Takes up the “correct” amount of space on large screens while still allowing you to compare sizes of all your sprites/sprite size relative to stage
• Minimize "dead space" in bitmap paint editor

Cons:

• More opportunities for weird non-integer scaling ratios: “why does my sprite look jagged/blurry?” This can be simulated on a low pixel density device by pressing the '+' button a few times

Design

• Add the zoom to fill button in addition to the 100% button to not lose functionality
• What does the button look like?
• This is not a “mode” where fitting sticks when you resize the browser, just a one-time zoom change (for simplicity)

/cc @carljbowman @kathymakes @kchadha