Modified IOptionsPage interface for Apply button

The previous interface wasn't sufficient to properly support the Apply
button, since it didn't separate applying the changes from the cleanup
phase.

Designer settings pages still need some fixing.

Fixes Apply button causing problems for certain plugins, for example the
QuickOpen.

src/plugins/git/settingspage.cpp

@@ -95,11 +95,10 @@ QWidget *SettingsPage::createPage(QWidget *parent)
     return m_widget;
 }
 
-void SettingsPage::finished(bool accepted)
+void SettingsPage::apply()
 {
-    if (!accepted || !m_widget)
+    if (!m_widget)
         return;
 
     GitPlugin::instance()->setSettings(m_widget->settings());
 }
-