Make the overlay toggable

data/de.wolfvollprecht.UberWriter.gschema.xml

@@ -13,6 +13,14 @@
         asked to install them manually.
       </description>
     </key>
+    <key name='gradient-overlay' type='b'>
+      <default>true</default>
+      <summary>Show gradient overlay</summary>
+      <description>
+        Show a gradient overlay over the text at the top anf bottom of the window.
+        It can cause performance problems to some users
+      </description>
+    </key>
     <key name='open-file-path' type='s'>
       <default>"/tmp"</default>
       <summary>Open file base path</summary>

uberwriter/UberwriterWindow.py

@@ -146,7 +146,12 @@ class UberwriterWindow(Gtk.ApplicationWindow):
         self.scrolled_window.add(self.text_editor)
         self.alignment_padding = 40
         self.editor_viewport = self.builder.get_object('editor_viewport')
-        self.scrolled_window.connect_after("draw", self.draw_gradient)
+
+        # some people seems to have performance problems with the overlay. 
+        # Let them disable it
+        
+        if self.settings.get_value("gradient-overlay"):
+            self.scrolled_window.connect_after("draw", self.draw_gradient)
 
         self.smooth_scroll_starttime = 0
         self.smooth_scroll_endtime = 0