Grey out Apply button when there are no changes

Resolves #1313

What this commit does:
* Whenever the Apply button is pressed, and if the save was successful, then the Apply button is disabled.
* Each subwidget used by EditEntryWidget has now a signal called `widgetUpdated` that is emitted when the widgets' internal content changes. The EditEntryWidget subscribes to that signal to know when to enable the Apply button (by calling `entryUpdated()`).
* There are some views that are not isolated in their own widgets (`m_advancedUi`, for example) so in those cases I invoked `entryUpdated()` directly whenever I detected an update:
  * some updates occur directly in a Qt widget like when editing the text of a QLineItem, so in that case I connected the widget's signals directly to the `entryUpdated()` slot.
  * some updates occur in EditEntryWidget, so in those cases the invocation to `entryUpdated()` is made as soon as the change is detected (for example when the user has confirmed an action in a dialog).

A known problem: there are some situations when the Apply button will get enabled even if there are no changes, this is because the app changes the value of a field by itself so it's considered an update (for example, clicking on the "Reveal" button changes the text shown in a text field).
The solution to this can be a bit complicated: disabling temporarily the `entryUpdated()` whenever the app is going to do an action with such side-effects.
So I preferred to let the Apply button get enabled in those cases.

src/gui/EditWidgetIcons.h

@@ -62,6 +62,7 @@ public slots:
 signals:
     void messageEditEntry(QString, MessageWidget::MessageType);
     void messageEditEntryDismiss();
+    void widgetUpdated();
 
 private slots:
     void downloadFavicon();