VFS: cleaner code for refresh check on vfs operation without current directory

Summary:
Rework of filesystem info and space update after refresh and file operation. Part of cleaning VFS
BUG: 374404

VFS: cleaning inheritance issues with VFS classes. Part of cleaning VFS

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The bug report https://bugs.kde.org/show_bug.cgi?id=374404 made me notice some flaws with the current file space refresh. I couldn't help myself and changed most of the code.

Behaviour changes:
* free space update should work now flawless. I.e. if changes to the filesystem space are made inside Krusader all panels (and tabs) showing the same filesystem are updated
* no special label for /etc and /proc anymore. I feel its fine to show "0 of of 0 bytes" here. And /sys was missing anyway.
* "Virtual Filesystem" is shown now if "virt_fs" is used (and no search meta information is present).

Test Plan:
Checked panels filesystem stats update in both panels (+tabs) when
* copying file on local fs with {F5/copy, drag&drop, copy/paste}
* deleting file on local fs
* creating virtual directory from search result ("feed to listbox")
* changing directory/refreshing to different fs

Reviewers: #krusader, martinkostolny, asensi

Reviewed By: #krusader, martinkostolny, asensi

Subscribers: asensi, martinkostolny

Differential Revision: https://phabricator.kde.org/D3938