Synchronize the SwtResourceManager with the JFace ResourceManager

There is quite a lot of overlap between both classes. We should try to extend the JFace class in order to reduce the amount of duplicate code.

Following methods should be compatible:
`getColor(int)` -> `Display.getSystemColor(int)`
`getColor(int, int, int)`, `getColor(RGB)` -> `createColor(ColorDescriptor)`
`getImage(int)` -> `Display.getSystemImage(int)`
`getImage(String)`, `getImage(Class, String)`-> `createImage(ImageDescriptor)`
`getFont(...)` -> `createFont(FontDescriptor)`

The difficult method is going to be the `decorateImage()` method, which we use to quickly construct composite images. JFace doesn't provide a function for such a specific use-case.