scribus-qt5: update to 1.5.8

Like previous releases, work on version 1.5.8 focused mostly on bugfixes and
code optimizations, the latter of which are expected to lead to reliability
and speed improvements. We have also begun to prepare Scribus for using Qt6,
which will make our work easier for future releases.

High Level Changes:
* UI Improvements for dark mode and some icon updates and window interactivity
* Improvements to file import (IDML, PDF, PNG, TIFF, SVG)
* Improvements to PDF export
* Improvements with respect to tables (undo/redo, styles)
* Improvements to the Story Editor
* Improvements to the build system
* Translation updates
* On mac OS, Scribus now includes a bundled Python 3 interpreter and is built
  for mac OS 10.15/Catalina or higher.

1.5.7
Significant effort went into clean up of user interface margins and widget
sizing, however most changes have taken place ���under the hood���. Apart from bug
fixing this also meant to refine previously introduced features, as well as
preparing the code for future Qt versions. A few remaining issues aside, this
release has been thoroughly tested and is considered to be stable. However,
there are also some new features.

(markd)