screenshot: Add PickColor() method to Screenshot interface

Use the newly added API to implement a color picker method in the
screenshot interface, so that the desktop portal can expose it
to applications.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/286

data/org.gnome.Shell.Screenshot.xml

@@ -91,6 +91,23 @@
       <arg type="s" direction="out" name="filename_used"/>
     </method>
 
+    <!--
+        PickColor:
+
+        Picks a color and returns the result.
+
+        The @result vardict contains:
+        <variablelist>
+          <varlistentry>
+            <term>color (ddd)</term>
+            <listitem><para>The color, RGB values in the range [0,1].</para></listitem>
+          </varlistentry>
+        </variablelist>
+    -->
+    <method name="PickColor">
+      <arg type="a{sv}" direction="out" name="result"/>
+    </method>
+
     <!--
         FlashArea:
         @x: the X coordinate of the area to flash