Add simple lock file per configuration

Make use of lock files in configuration directory
for preventing multiple instances of Mirage from
being running at the same time, at least for a
single configuration.
Running multiple instances of Mirage is known to
cause encryption errors in one or more instances.

src/gui/Window.qml

@@ -109,10 +109,12 @@ ApplicationWindow {
         tooltip: qsTr("Mirage")
         icon.source: `../icons/${iconPack}/tray-icon.png`
 
+        onActivated: window.show()
+
         menu: Menu {
             MenuItem {
                 text: qsTr("Quit")
-                onTriggered: window.close()
+                onTriggered: window.hide()
             }
         }
     }