fix: config files for themes are not found

[ThorstenSuckow]

themes do not have a separate folder. As a workaround, the config file can be placed in the "coon-js" folder of the owning package, while still omitting this folder from the config-path, i.e. ${coon-js.resourcePath}/theme.conf.json. The coon.core- application-class should consider this and automatically look up config files for themese in the appropriate folder