* Core: Introduce PDF Import-Export dialog
Currently, we only have hardcoded exporting to one pdf version, which
leads to (as in one of the below issues) to problems, such as
non-transparent layers, which are by default not supported in PDF/A-1b.
So, this patch adds an option to the user to select their preferred PDF
version by introducing new dialog under Import-Export in Preferences,
and allowing to update that version before exporting.
* Core: Add informational messages regarding selected PDF format
* Gui: Handle Qt < 6.8 unsupported PDF format
* Gui: Switch PDF export prefs dialog to use tr()
* Gui: Remove center alignment of PDF version description
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Co-authored-by: Chris Hennes <chennes@pioneerlibrarysystem.org>
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