Add switch_greekmath to the extra keys

The Fn+switch_numeric shortcut might be hard to discover or too slow for
heavy users.

A location is allocated for the key so it has a consistent placement.

res/xml/bottom_row.xml

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <row height="0.95">
-  <key width="1.8" key0="ctrl" key2="meta" key4="switch_numeric"/>
+  <key width="1.8" key0="ctrl" key1="loc switch_greekmath" key2="meta" key4="switch_numeric"/>
   <key width="1.2" key0="fn" key1="alt" key2="change_method" key3="switch_emoji" key4="config"/>
   <key width="4.0" key0="space" key1="switch_programming" edgekeys="true"/>
   <key width="1.2" key1="up" key2="right" key3="left" key4="down" edgekeys="true"/>

res/xml/settings.xml

@@ -20,6 +20,7 @@
       <juloo.keyboard2.ExtraKeyCheckBoxPreference app:index="12"/>
       <juloo.keyboard2.ExtraKeyCheckBoxPreference app:index="13"/>
       <juloo.keyboard2.ExtraKeyCheckBoxPreference app:index="14"/>
+      <juloo.keyboard2.ExtraKeyCheckBoxPreference app:index="15"/>
     </PreferenceScreen>
   </PreferenceCategory>
   <PreferenceCategory android:title="@string/pref_category_typing">

srcs/juloo.keyboard2/ExtraKeyCheckBoxPreference.java

@@ -28,7 +28,8 @@ public class ExtraKeyCheckBoxPreference extends CheckBoxPreference
     "accent_ogonek",
     "€",
     "ß",
-    "£"
+    "£",
+    "switch_greekmath",
   };
 
   boolean _key_font;