Update to v1.5.1

This update contains the following changes:

• Added new Birchgen tab - makes birch-style trees
• Added more palettes
• Removed reliance on external palette images; all palette handling is contained internally via worker script
• Improved preview rendering logic - program now renders tree previews in full resolution, then scales down
• Minor tweaks to pine tree generation and setting logic
• Removed reliance on external output counting file; exported vox files are now titled via timestamp
• Added multiprocessing for all tree generation logic and previewing logic for quicker response times
• Sliders in all tabs are now appropriately themed

Thank you for using my free program!