Allow scrolling on notebook tabs to change the active page

This is enabled by default but has a setting in the preferences so it can be
disabled.

data/org.x.editor.gschema.xml.in

@@ -126,6 +126,11 @@
       <summary>Bottom Panel is Visible</summary>
       <description>Whether the bottom panel at the bottom of editing windows should be visible.</description>
     </key>
+    <key name="enable-tab-scrolling" type="b">
+      <default>true</default>
+      <summary>Allow changing active tabs by scrolling</summary>
+      <description>Whether you can change active tabs by scrolling.</description>
+    </key>
     <key name="max-recents" type="i">
       <default>5</default>
       <summary>Maximum Recent Files</summary>