Multi Molecule Image Service

[Mailaender]

This fixes a collection of issues arising for having more than one image service since #1286 and #1288.

[eselmeister]

This change looks good, but it leads to an error in the OpenChrom/CDK package:
net.openchrom.xxd.identifier.supplier.cdk.ui.services

Description	Resource	Path	Location	Type
The type MoleculeImageService must implement the inherited abstract method IMoleculeImageService.isOnline()	MoleculeImageService.java	/net.openchrom.xxd.identifier.supplier.cdk.ui/src/net/openchrom/xxd/identifier/supplier/cdk/ui/services	line 36	Java Problem

Please create a PR in the OpenChrom/CDK package, so that both PRs can be merged in sync. Additionally, a public abstract class could be added, which defines "isOnline" by default as false. Any implementation, that uses online services, shall override to true.

public abstract class AbstractMoleculeImageService implements IMoleculeImageService

[Mailaender]

I can push to @OpenChrom without your help, so this shouldn't be necessary. Creating an AbstractMoleculeImageService only to default it to something that may be wrong and has to be overwritten sounds like bad design.