NPanel: add drag support

Modules/SettingsPanel/SettingsPanel.qml

@@ -21,6 +21,8 @@ NPanel {
 
   panelKeyboardFocus: true
 
+  draggable: true
+
   // Tabs enumeration, order is NOT relevant
   enum Tab {
     About,

Modules/WallpaperSelectorPanel/WallpaperSelectorPanel.qml

@@ -18,6 +18,8 @@ NPanel {
   panelAnchorVerticalCenter: true
   panelKeyboardFocus: true
 
+  draggable: true
+
   // Local reactive state
   property list<string> wallpapersList: []
   property string currentWallpaper: ""