Add allNames() method to PropertyInfo

Currently we kinda have ~3 slightly different implementations regarding metadata processing between Dolphin/KFileMetaData/Baloo/BalooWidgets.

I have been working on improvements for Dolphin to remove some duplication regarding this.

Once everything will be added it will look like this: Dolphin => BalooWidgets => KFileMetaData/Baloo

To reduce this duplication there will be FileMetaDataRoles in BalooWidgets that will use this. Like

Baloo::FileMetaDataRoles::FileMetaDataRoles()
{
    auto names = KFileMetaData::PropertyInfo::getAllNames();
    // ...
}

And KBalooRolesProvider (https://invent.kde.org/system/dolphin/-/blob/master/src/kitemviews/private/kbaloorolesprovider.cpp#L36) will be removed from Dolphin

This is the first MR from all my patches :)

cc @meven