Provide JSON Schema-Support

[de-jcup]

https://github.com/java-json-tools/json-schema-validator

Schemafiles should be very simple to assign:

a) context menu "Set Yaml/JSON Schema"
b) main toolbar via a dedicated button/command

There shall a dialog appear with
a) List of all wellknown schemas
b) Show current selected schema (if selected)
c) Have a "NONE" entry as well
d) show all schemas available at ~/.de-jcup.schemas/ (or still ~/.yaml-editor/schemas/)
e) provide selection
f) provide an import of new schemas
g) The possiblity to add
h) have standard schemas "on board"

this should be same implementation as in de-jcup/eclipse-yaml-editor#61