Fix Tree rendering and color reset on Windows

• Trees (such as the Project Explorer) would no longer render properly in Eclipse 2024-06 running on Windows. This has now been fixed.
• Editor colours would occasionally reset when saving files on Windows. This has now been fixed.
• Performance of the themes has been improved.
• As a downside to above fixes, the flat scrollbar functionality provided by Xiliary on Windows has been dropped. Whilst a great addition to the themes, the growing number of issues and the maintenance overhead made it difficult to keep them around.
• Fix source code hover colour mixup between the Moon and Neptune themes.