Add mounter abstraction, enable minimal GVfs support

examples/gvfs.rs

@@ -0,0 +1,25 @@
+use gio::prelude::*;
+
+fn main() {
+    let monitor = gio::VolumeMonitor::get();
+    for drive in monitor.connected_drives() {
+        println!("Drive: {}", drive.name());
+        for volume in drive.volumes() {
+            println!("  Volume: {}", volume.name());
+            if let Some(mount) = volume.get_mount() {
+                println!("    Mount: {}", mount.name());
+            }
+        }
+    }
+
+    for mount in monitor.mounts() {
+        println!("Mount: {}", mount.name());
+    }
+
+    for volume in monitor.volumes() {
+        println!("Volume: {}", volume.name());
+        if let Some(mount) = volume.get_mount() {
+            println!("  Mount: {}", mount.name());
+        }
+    }
+}