Slow down of first context menu due to ELK loading services

[Bananeweizen]

Describe the bug
Opening the first context menu in the package explorer/navigator of an application containing ELK leads to a notable delay before the menu becomes visible. ELK causes half a second of delay by loading layout services when it's property tester is used the first time:

Expected behavior
Bundle activators should do as little work as possible, as bundle activation can be triggered by UI interaction and will then slow down everything happening on the UI thread.

ELK Version
0.9.0

Additional context
Looking at the relevant method at

elk/plugins/org.eclipse.elk.core.service/src/org/eclipse/elk/core/service/ElkServicePlugin.java

Line 136 in f53099f

private void loadLayoutProvidersThroughJavaServices() {

it seems like all of that could be executed inside of LayoutMetadataService.getInstance() instead, which would defer the code until it's needed, and the plugin activation would be fast again.