Bug fixes:
- Set label background to avoid overlapping text if theming is disabled
- Trim inline editor text to avoid "invisible" breakpoints
- Add more space for inline editor if original labels is very short

Refactoring:
- perform preconditions checking at the beginning
- extracted repeated dispose() code to InlineEditor.dispose()
- extract Mac workaround code to dedicated method
- extract editor extent calculation to dedicated method
- removed useless tree.setText(current) and tree.setText("") calls
- don't use syncExec to get RGB of red color

Fixes https://github.com/eclipse-platform/eclipse.platform/issues/2262