Allow use custom icon

In case you use gin as base theme for your admin theme, it is not enough to get setting of gin theme directly, you need to get the setting of current admin theme. The patch includes this and also the change from the template for menu toolbar from related issue https://www.drupal.org/node/3121437

Fixed typo in function name

Hey thanks to you both for your work on this! It's on my todo list for reviewing :)
But this should go into the Gin theme itself as this module is really just a wrapper module which injects styles into the frontend when you're logged into Drupal. Therefore can we close this as a duplicate to #3121437: Custom logo in toolbar?
Thanks again :)
Sorry I just saw this enables the feature, sorry my fault.

The patch is needed if your admin theme uses Gin theme as base theme and also provides the variables for toolbar template.

Updated patch to match the new patch in the main ticket.

@saschaeggi why do you force the theme setting to be from 'gin' theme only? what if there is custom admin theme that is based on gin? With the patch from #3 or #5 it is still possible to use any custom theme based on gin. As in this issue https://www.drupal.org/project/gin_toolbar/issues/3139152 we make sure that Gin is installed, it is safe to have theme_settings not restricted to 'gin'

Sure we can check that, so you can subtheme Gin as well.
New patch attached.

Thanks, for adding this in, but there are more settings in gin_toolbar_preprocess_html :) Please also add them as well. You can check patch in #5 as a reference.

@a.dmitriiev updated the patch for subtheming. Please review

Tested by breidert in #3121437: Custom logo in toolbar

Included in dev, thanks for your effort!

Thank you @saschaeggi, great job!