Dialog engine stuff: see my Patreon post for more information

Other things since the last build:

• Tweaks to the main menu (fonts, layout, etc.)
• Finally set up an app icon for the macOS build
• Added luacheck to the build and fixed all the issues it found (which included some actual code bugs I hadn't noticed! including some subtle physics issues in Little Bouncing Ball)
• Made shaders a pooled, collected resource rather than always having them all loaded all the time