Add KDE theming support

[ilya-fedin]

KDE applications are a notable part of the popular Qt software in the Linux world. Sadly, they have their own theming add-ons that are applied through QPlatformTheme.

This adds support for all aspects of the KDE applications theming present in the plasma-integration QPlatformTheme:

• KDE's QtQuick style bridging the currently set QtWidgets style (you may want to get QtQuick style choice in the settings UI in the future instead)
• KDE's color schemes (the most important part as KDE applications don't fallback to QtQuick style/QtWidgets style/QPalette for colors)
• KDE's icon engine applying KDE's color schemes to icons (to not to have black icons on black background)

I haven't implemented preview for KDE color schemes as I don't really need it. I can try to make the preview part of the dialog to be hidden when a KDE color scheme is chosen or I can leave it as it is if you wish to implement it later.

I also have a qt5ct variant of this patch but I don't have a SourceForge account nor SVN experience to send it there... If you wish I can upload the patch here as a file.