Allow Eclipse project-specific settings to be merged

[wilkinsona]

The Eclipse plugin allows custom project-specific settings to be provided. I'd like to customise a single setting (org.eclipse.jdt.ui.importorder in org.eclipse.jdt.ui.prefs) to align with configuring Checkstyle with a custom project root package. However, any custom settings file is used instead of the plugin's default org.eclipse.jdt.core.prefs and org.eclipse.jdt.ui.prefs files so I have to copy the plugin's defaults and modify them as needed. This has to be redone each time the plugin is upgraded to avoid missing changes to the defaults.

I'd like to be able to provide an Eclipse settings file that is merged on to the plugin's defaults. This should make it easier to use the plugin's defaults across upgrades and also make it clear exactly which settings are being customised.