implement issue #11: Remember color history between sessions

data/com.github.ronnydo.colorpicker.desktop

@@ -1,6 +1,6 @@
 [Desktop Entry]
 Name=Color Picker
-Comment=The one and only Color Picker to speed up professionals workflow.
+Comment=An intuitive Color Picker
 Exec=com.github.ronnydo.colorpicker
 Icon=com.github.ronnydo.colorpicker
 Keywords=color;picker;selector;pipette;

data/com.github.ronnydo.colorpicker.gschema.xml

@@ -13,5 +13,10 @@
             <summary>Most recent y position of app window</summary>
             <description>Most recent y position of app window</description>
         </key>
+        <key name="color-history" type="as">
+	        <default>[]</default>
+	        <summary>Color History</summary>
+	        <description>Color History</description>
+        </key>
     </schema>
 </schemalist>