Extended Eclipse version compatibility and various improvements

• All themes are now compatible with a wide range of older Eclipse versions, and can be used on releases as old as Eclipse Oxygen (4.7).
• Visual glitches in Eclipse 2020-12 have been fixed, in particular selected tab outlines and styling of some Java keywords.
• Starting with Eclipse 2020-12, trees and tables will no longer display white lines in Pluto and Neptune on Windows.
• The line deletion color in Pluto has been changed to purple to avoid confusion with warnings in the overview ruler.
• Unselected tabs now feature a subtle hover effect, making the cursor position more obvious to users.
• Rendering performance has been slightly improved, Planet themes are now expected to be snappier that Eclipse's built-in ones!