Limit save directory path to alphanumeric plus underline, dash & /

This can be localized for non qwerty characters, but want to keep it
simple.  (Ideally a file picker should be used along with the Storage
Access Framework so that files can be saved anywhere, including to cloud
services.)

src/main/res/values/strings.xml

@@ -159,4 +159,8 @@
     <string name="config_storage_path">Storage Folder Path</string>
     <string name="config_storage_page_hint">Where captured media is stored</string>
 
+    <!-- accepted digits for directory name -- may vary for locale/platform/etc.
+         Skipping periods, spaces, and other special characters to keep it simple -->
+    <string name="path_chars">0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-_/</string>
+
 </resources>

src/main/res/xml/settings.xml

@@ -12,6 +12,7 @@
         android:summary="@string/config_storage_page_hint"
         android:title="@string/config_storage_path"
         android:defaultValue="/phoneypot"
+        android:digits="@string/path_chars"
         />
 
     <Preference