[wallpapers/image] Wallpapers ordering by folders and case sensitivity

Added ability to order the images by path and case sensitively/insensitively to Desktop Settings/Wallpapers Slideshow plugin:

• A-Z Folders first - the folders subfolders first, files last, folders ordered also A-Z
• A-Z Folders last - the folders files first, subfolders last, folders ordered also A-Z
• A-Z Full path - the folders and files ordered A-Z
• Z-A Folders first - the folders subfolders first, files last, folders ordered also Z-A
• Z-A Folders first - the folders subfolders first, files last, folders ordered also Z-A
• Z-A Full path - the folders and files ordered Z-A
• A-Z Files only - files ordered A-Z, regardless the parent folder
• Z-A Files only - files ordered Z-A, regardless the parent folder

The case sensitivity is taken into account for the above cases.

[Solved] I can't properly test due to a problem I need assistance with:

During testing, using kdesrc-build to build and install in my KDE Neon setup, I get a different theme from the custom built installation, so the UI looks flat and the controls horizontally centered. I've tried to access the settings, but I get the following:

systemsettings5: symbol lookup error: /home/myuser/kde/usr/lib/x86_64-linux-gnu/plugins/classic_mode.so: undefined symbol: _ZN8BaseModeC2EP7QObject