Center padding option for Surface Duos/other foldables (#634)

* Add center margin option for horizontal dual page view

* Update readme to make the point of the fork obvious

* Center margins now added to large single images that would display in place of two smaller pages

* Cleanup; reworked preference into an int selector to specify which margins to add

* Suggested fixes and and readme reversion which somehow got skipped last commit

* Missed the build block in readme. Oops.

app/src/main/res/values/arrays_sy.xml

@@ -21,4 +21,11 @@
         <item>@string/double_pages</item>
         <item>@string/automatic_orientation</item>
     </string-array>
+
+    <string-array name="center_margin_types">
+        <item>@string/center_margin_none</item>
+        <item>@string/center_margin_double_page</item>
+        <item>@string/center_margin_wide_page</item>
+        <item>@string/center_margin_double_and_wide_page</item>
+    </string-array>
 </resources>

app/src/main/res/values/strings_sy.xml

@@ -314,6 +314,15 @@
     <string name="automatic_can_still_switch">While using automatic page layout, you can still switch between layouts while reading without overriding this setting</string>
     <string name="invert_double_pages">Invert double pages</string>
 
+    <!-- Center margin -->
+    <string name="center_margin">Center Margin</string>
+    <string name="center_margin_none">None</string>
+    <string name="center_margin_double_page">Add to double Page</string>
+    <string name="center_margin_wide_page">Add to wide Page</string>
+    <string name="center_margin_double_and_wide_page">Add to both</string>
+    <string name="pref_center_margin">Center margin type</string>
+    <string name="pref_center_margin_summary">Insert spacer to accommodate deadspace on foldable devices.</string>
+
     <!-- Manga Page -->
     <!-- Manga Info -->
     <string name="az_recommends">See Recommendations</string>