This should retain the smoother look introduced by the gamma correction
changes in the 4.19 cycle, as well as the rounding jitter that fixes
banding, but appears to run in about 10% of the time the old code did.

Closes #347

See https://mesonbuild.com/Reference-manual_functions.html#project_license

Sometime after xfdesktop's clipboard manager was copied from Thunar,
Thunar's behavior changed to send path names instead of file: URIs when
copying to the clipboard & the receiver requests a string
representation.

This makes the behavior consistent with the current version of Thunar,
and cleans up the code a little bit.

Closes #378

This also ensures that XfceDesktop->single_workspace_num always gets set
to a valid value, even if it's not set in xfconf.

Closes #379

Closes #384

Closes #376

Closes #381

When gdk_pixbuf_loader_write() or gdk_pixbuf_loader_close() fails, the
'closed' signal is still emitted, and in that handler, we clean things
up and free the image data, so by the time those functions return FALSE,
we can't touch it anymore.

This fixes an issue where thumbnails become available while the image
directory is still being enumerated, before the model is set on the icon
view, and the thumbnail ready callback tries to get the model from the
icon view.

This also changes the behavior when an inaccessible folder is selected:
now it will properly revert to the previous folder without changing
anything else.

Closes #386

If the user has used the button to select a folder, then
gtk_file_chooser_set_current_folder() stops working for some reason.

xfw_windowing_get() requires that GDK is initialized.

Related to #367.

Closes #367.

Related: xfce4-dev-tools!145

Closes #374

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