Merging style and icons tabs into a theme tab

Submitted by Yousuf Philips `@philipzae`

Assigned to Xfce Bug Triage

Created attachment 9532 how it will look

Rather than users having to jump between tabs to finalize tweaking of their theme, we merge the style and icon tabs into a theme tab.

Attachment 9532, "how it will look":

Version: 4.15.0