Add an example for gio::Vfs implementation

Signed-off-by: fbrouille <fbrouille@users.noreply.github.com>

Author: fbrouille

Cargo.lock

@@ -51,3 +51,8 @@ path = "object_subclass/main.rs"
 [[bin]]
 name = "virtual_methods"
 path = "virtual_methods/main.rs"
+
+[lib]
+name = "gio_vfs"
+path = "gio_vfs/lib.rs"
+crate-type = ["cdylib"]

examples/Cargo.toml

@@ -0,0 +1,53 @@
+# GIO VFS
+
+This example demonstrates the usage of GIO VFS. Built artifact is a dynamic system library that is used as a GIO module
+(see https://docs.gtk.org/gio/overview.html#running-gio-applications)
+
+Build, install and configure it by executing:
+```bash
+cargo build -p gtk-rs-examples --lib
+export GIO_EXTRA_MODULES=/tmp/gio_modules
+mkdir -p $GIO_EXTRA_MODULES && cp ./target/debug/libgio_vfs.so $GIO_EXTRA_MODULES
+export MYVFS_ROOT=/tmp/myvfs
+mkdir -p $MYVFS_ROOT
+```
+
+`GIO_EXTRA_MODULES` specify additional directories for `gio` command line tool to automatically load modules.
+
+`MYVFS_ROOT` specify the local directory that is used by as backend directory for uri starting with `myvfs:///` (e.g. if `MYVFS_ROOT-/tmp` `myvfs:///foo` points to `/tmp/foo`).
+
+`gio` command line tool automatically loads this extra module that implement all file commands for files with uri starting with `myvfs:///`
+
+Run it by executing the following commands:
+
+Basic operations:
+```bash
+echo "foo" | gio save myvfs:///foo
+gio cat myvfs:///foo
+gio set -t string myvfs:///foo xattr::my_string value
+gio info myvfs:///foo
+gio mkdir myvfs:///bar
+gio copy myvfs:///foo myvfs:///bar/
+gio list myvfs:///
+gio tree myvfs:///
+gio move -b myvfs:///bar/foo myvfs:///foo
+gio tree myvfs:///
+gio remove myvfs:///foo myvfs:///foo~ myvfs:///bar
+gio list myvfs:///
+```
+
+Monitor `myvfs:///`:
+```bash
+# monitor is a blocking operation. kill it with Ctrl+C
+gio monitor myvfs:///
+```
+
+```bash
+# in another terminal (ensure MYVFS_ROOT is defined)
+touch $MYVFS_ROOT/foo
+echo "foo" > $MYVFS_ROOT/foo
+mkdir $MYVFS_ROOT/bar
+cp $MYVFS_ROOT/foo $MYVFS_ROOT/foo2
+mv -b $MYVFS_ROOT/foo2 $MYVFS_ROOT/foo
+rm -rf $MYVFS_ROOT/*
+```