Multiselect: Create 'select all' in public and secret keylist

OpenPGP-Keychain/src/main/res/menu/key_list_public_multi.xml

@@ -1,6 +1,10 @@
 <?xml version="1.0" encoding="utf-8"?>
 <menu xmlns:android="http://schemas.android.com/apk/res/android">
 
+    <item
+        android:id="@+id/menu_key_list_public_multi_select_all"
+        android:icon="@drawable/ic_action_select_all"
+        android:title="@string/menu_select_all" />
     <item
         android:id="@+id/menu_key_list_public_multi_encrypt"
         android:icon="@drawable/ic_action_secure"

OpenPGP-Keychain/src/main/res/menu/key_list_secret_multi.xml

@@ -1,6 +1,10 @@
 <?xml version="1.0" encoding="utf-8"?>
 <menu xmlns:android="http://schemas.android.com/apk/res/android">
 
+    <item
+        android:id="@+id/menu_key_list_public_multi_select_all"
+        android:icon="@drawable/ic_action_select_all"
+        android:title="@string/menu_select_all" />
     <item
         android:id="@+id/menu_key_list_public_multi_delete"
         android:icon="@drawable/ic_action_discard"

OpenPGP-Keychain/src/main/res/values/strings.xml

@@ -96,6 +96,7 @@
     <string name="menu_beam_preferences">Beam settings</string>
     <string name="menu_key_edit_cancel">Cancel</string>
     <string name="menu_encrypt_to">Encrypt to…</string>
+    <string name="menu_select_all">Select all</string>
 
     <!-- label -->
     <string name="label_sign">Sign</string>