Support images in LSP that are located outside of bundles

[a-sr]

Currently, the Klighd LSP only supports KImages that refer to images in a bundle, hence the LS jar. This should be extended to support any URI in order to enable images defined by the modeller, e.g. on the file system.

https://github.com/kieler/KLighD/blob/013d6687ead7fee7041eff36f98e02375e5b2b56/plugins/de.cau.cs.kieler.klighd.lsp/src/de/cau/cs/kieler/klighd/lsp/model/DiagramMessageData.xtend#L44-L54