feat: if select has display:none save style

[rkteam]

if source select has display:none select2 will have the same style.
Add watcher to source select2

[ishikawa999]

@rkteam

Thank you for your thoughtful review and sorry for the delayed response.

Regarding the change to use MutationObserver, I think it is a great change that solves the problems we have been having. Although MutationObserver is an efficient tool to observe changes in DOM, it might indeed affect performance if there are too many select boxes on the page, especially on lower-spec machines. For example, a previous change to Redmine, which added tooltips to the workflow screen, slowed it down in some environments. https://www.redmine.org/issues/34247

What about making it so that it only works if the user enables the plugin's settings, such as Support Built-In / Custom Fields in the redmine_issue_templates plugin?
This way, users can choose to enable or disable this functionality based on their system's functionality and specific needs, and if enabling it causes problems, they can disable it.

Many thanks,