Allow plugin controls to be configured using the main configuration file

[sweco-sedalh]

Is your feature request related to a problem? Please describe.
Currently, configuration for plugin controls must be handled separately, usually by embedding their configuration in the HTML/JS-files. This is less than ideal as it results in having to write separate HTML/JS-files if you have multiple maps that need different configurations of the plugin controls.

Describe the solution you'd like
Built-in controls are configured using the normal configuration file. Plugin controls could be configured the same way, just using controls provided by the instantiator rather than Origo itself.

Describe alternatives you've considered
The same code could be implemented externally to Origo, but this is something that many could find useful.

Instead of combining the configuration with the built-in controls a separate configuration object ("plugins") could have been used, but from the PoV of the configuration editor there isn't really a difference between built-in and plugin controls.

Additional context