Add migrate process form plugin for "static_map"

Additionally, I also made the following improvements:

• Category and label may now also come from the form plugin. This way the selector displays "Static map" instead of "static_map" and it is listed underneath "Text" instead of "Other".
• The form plugin for "default_value" has been updated accordingly: label changed slightly and the plugin is now listed underneath the category "Text".

Tests are passing! Merged the code.

Automatically closed - issue fixed for 2 weeks with no activity.