Add (mounted) VeraCrypt file containers to the places sidebar

Originally created by @segfault on #15036 (Redmine)

Some devices, for example unlocked LUKS devices, are shown in the places sidebar of Nautilus and file chooser dialogs. Unlocked VeraCrypt volumes should also be shown there. This works out of the box for unlocked external devices, but not for file containers mounted via loop devices.

The places sidebar is part of GTK, so we will have to patch GTK for this.

Parent Task: #14464 (closed)

Related issues

Is duplicate of #15225 (closed)

Edited May 15, 2020 by segfault

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