Minor UI changes.

- The 'save' button is disabled and shows 'No changes' at first. Editing contentEdit's text
enables the button and makes it show 'Save'.
 - The 'save' button is disabled and shows 'Saving...' when the page is being saved, and stays
disabled until contentEdit's text has been changed again.
 - The 'save' button shows 'Saved' once the page has been successfully saved. fixes #560
 - contentEdit is readonly when the page is being saved

weboob/applications/qwebcontentedit/ui/main_window.ui

@@ -6,8 +6,8 @@
    <rect>
     <x>0</x>
     <y>0</y>
-    <width>469</width>
-    <height>520</height>
+    <width>498</width>
+    <height>543</height>
    </rect>
   </property>
   <property name="windowTitle">
@@ -89,7 +89,7 @@
          <bool>false</bool>
         </property>
         <property name="text">
-         <string>Save</string>
+         <string>No changes</string>
         </property>
        </widget>
       </item>
@@ -102,8 +102,8 @@
     <rect>
      <x>0</x>
      <y>0</y>
-     <width>469</width>
-     <height>20</height>
+     <width>498</width>
+     <height>22</height>
     </rect>
    </property>
    <widget class="QMenu" name="menuFile">