Fix loading custom properties into Tinymce skin

[tvdeyen]

Same as #3070 but for main branch.

For the Tinymce skin we need our custom properties available. We cannot simply compile them into the skin, because it gets loaded from Tinymce into the current document and this would lead to duplicated custom properties.

So instead we compile it as separate file and include it in the Alchemy admin as first stylesheet. This has the additional advantage that we can load this file into ie. Solidus to display the Alchemy Tinymce skin as well.