Centralize remaining theme-related options (like Rstudio does) under a renamed "Themes" pref pane

[CAM-Gerlach]

Problem Description

As originally discussed in #8020 and part of the broader #8068 Epic (fully adding a dark theme to Spyder), following the example of Rstudio (that we're apparently following for this) I propose we rename the Syntax coloring preference pane Themes and centralize the remaining theme-related options there, so users can easily find them in the most sensible place, and efficiently modify them without having to jump back and forth. It would make good use of the space to the left of the preview pane, while making the long and somewhat unfocused "General" preference pane less so.

Specifically, I propose moving the following settings which are directly theme-related, and/or have parallels in the Rstudio version:

• Plain text font
• Rich text font
• Qt Window Style
• Icon theme

These could either go below the current options (separated by e.g. a horizontal rule), or between "Spyder theme" and above "Scheme", with a horizontal rule between theme.