possible fix #75

I've added 'optimize' option (by default true). If it options is set
then sources will be checked if they are required (with dependencies if
any). And if they are not required they will be dropped by using common
mechanism.

Please note that if this option enabled the following features (at the
    moment) will be unavailable:

* key request from context menu (from configuration interface it still works)
* notifications event if sources on which notification is not connected

I suppose this commit will increase performance in about 4-5 times.

sources/awesome-widget/package/contents/ui/advanced.qml

@@ -45,6 +45,7 @@ Item {
     property alias cfg_wrapText: wordWrap.checked
     property alias cfg_notify: notify.checked
     property alias cfg_checkUpdates: updates.checked
+    property alias cfg_optimize: optimize.checked
     property alias cfg_height: widgetHeight.value
     property alias cfg_width: widgetWidth.value
     property alias cfg_interval: update.value
@@ -143,6 +144,20 @@ Item {
             }
         }
 
+        Row {
+            height: implicitHeight
+            width: parent.width
+            QtControls.Label {
+                height: parent.heigth
+                width: parent.width * 2 / 5
+            }
+            QtControls.CheckBox {
+                id: optimize
+                width: parent.width * 3 / 5
+                text: i18n("Optimize subscription")
+            }
+        }
+
         Row {
             height: implicitHeight
             width: parent.width