Support custom window backgrounds

[probonopd]

https://github.com/al45tair/ds_store/blob/master/doc/index.rst

Typically developers set an attractive background on their disk images, increase the icon size and font size and often include a link to the /Applications folder.

On the Mac, this kind of information is stored in .DS_Store files, which can be parsed, e.g., using the https://github.com/al45tair/ds_store tool written in Python.

[probonopd]

The Mac is using .DS_Store files to save properties of the window including icon size and position. Unfortunately that format is largely undocumented but has been reverse engineered to some extent, see https://metacpan.org/dist/Mac-Finder-DSStore/view/DSStoreFormat.pod. Seems rather complex and not all of it is understood. helloSystem is using a combination of extended attributes in the file system, and (optionally) .DirInfo files.

Trying to simplify things, we should possibly do something similar for backgrounds and just make filer use background files if present, similar to how it uses .DirIcon and .VolumeIcon.icns already. In Firefox 106.0.dmg, the file in question is stored in .background/background.png; however this is not standardized because on macOS, .DS_Store tells macOS where to load this from. Skype-8.89.0.403.dmg, for example, uses .hidden/background.tiff. Argh!

Inserting the following in void MainWindow::updateUIForCurrentPage() above updateViewMenuForCurrentPage(); almost works as intended:

    // Apply background
    qDebug() << "probono: tabPage->viewMode():" << tabPage->viewMode();
    // FIXME: This always seems to return 1, which is Fm::FolderView::IconMode
    if (tabPage->viewMode() == Fm::FolderView::IconMode || tabPage->viewMode() == Fm::FolderView::ThumbnailMode) {
        QString backgroundFile = tabPage->pathName() + "/.background/background.png";
        qDebug() << "probono: check for background:" << backgroundFile;
        if (QFile::exists(backgroundFile)) {
            QString ss = "background-image:url(" + backgroundFile +")";
            // qDebug() << "probono: ss:" << ss;
            tabPage->setStyleSheet(ss);
        }
    }

But:

• The background is always shown, not just in icon mode and thumbnail mode. This needs to be changed. tabPage->viewMode() seems to always return 1 (which means Fm::FolderView::IconMode). Why?
• MainWindow::updateUIForCurrentPage() apparently doesn't get called when the user switches the view mode. So probably this code needs to go elsewehre. Where?
• Not all (or at least most) real-world dmg files use .background/background.png as the location for window backgrounds. We would need to parse those dreaded .DS_Store files for full Mac background location support; as a stop-gap measure something like strings 'strings .DS_Store' | grep -i Background | grep ^/ | tail -n 1 might be a reasonably intelligent guess?

# Skype
FreeBSD% strings '.DS_Store' | grep -i Background | grep ^/ 
/.background/background.png

# Firefox
FreeBSD% strings '.DS_Store' | grep -i Background | grep ^/ 
/.hidden/background.tiff