Add background image replacement crop & resize tool

[WiIIiam278]

Closes #122

Adds a UI for resizing and cropping background images.

You can drag the image around, use ctrl+scroll to quickly scale, and use the arrowkeys to (or use the UI to do these things).

There's also a function for auto-scaling and resetting the image position, and it tracks the aspect ratio to make scaling a bit less painful.

Screenshots

Might still need a bit of code cleanup / tweaks here and there -- but feel free to review

[jonko0493]

This is so good!!

After testing on Windows, I have two notes:

• On Windows, the image does not show up until you attempt to resize it
• I would like for the Ctrl+Scroll & arrow keys controls to be documented in the window itself so that it's obvious those can be used

[jonko0493]

Same issues apply to Linux.

[WiIIiam278]

Should be fixed

[jonko0493]

Is fixed, but now one more issue:

the buttons are off the screen

[jonko0493]

Fantastic work. Let's merge it :)