Snippets: Add more use-cases

- Restored removed built-in snippets.
- Revert a built-in snippet.
- Reset the snippets collection.

src/plugins/texteditor/snippets/snippetssettingspage.ui

@@ -79,6 +79,27 @@
          </property>
         </widget>
        </item>
+       <item>
+        <widget class="QPushButton" name="revertButton">
+         <property name="text">
+          <string>Revert Built-in</string>
+         </property>
+        </widget>
+       </item>
+       <item>
+        <widget class="QPushButton" name="restoreRemovedButton">
+         <property name="text">
+          <string>Restore Removed Built-ins</string>
+         </property>
+        </widget>
+       </item>
+       <item>
+        <widget class="QPushButton" name="resetAllButton">
+         <property name="text">
+          <string>Reset All</string>
+         </property>
+        </widget>
+       </item>
        <item>
         <spacer name="verticalSpacer">
          <property name="orientation">