Added support for replacing skins through deltas

[vampcat]

Partially solution for the problems outlined in #3238.

Changes

Allows module creators to replace the skin for a particular UI screen by providing a delta file. Note that only support for skin has been added right now, and (if required) everything else will have to implemented separately.

How to test

(I'm considering you're developing Sample module, and want to replace the skin used in toolbar.ui (hud) with a custom skin (hudLarge))

• Checkout the PR
• Add the skin you want to replace an existing skin with (in my example, create \modules\Sample\assets\skins\hudLarge.skin)
• Add the delta for the UI skin you want to update (in my example, add a file \modules\Sample\deltas\engine\ui\hud\toolbar.ui with the following content: { "skin": "HUDLarge" })
• Run the game, and have the skin in toolbar.ui screen replaced from hud to hudLarge

[GooeyHub]

Hooray Jenkins reported success with all tests good!

[skaldarnar]

Tests out well. Would you please write some lines of Javadoc stating what you already said in the PR comment.

Otherwise, good to merge 👍

[GooeyHub]

Hooray Jenkins reported success with all tests good!