feat: add Shade/Unshade/ToggleShade actions

This builds on the work of @Consolatis in #1018.

Co-authored-by: Consolatis <35009135+Consolatis@users.noreply.github.com>
Co-authored-by: Andrew J. Hesford <ajh@sideband.org>

docs/labwc-actions.5.scd

@@ -229,6 +229,15 @@ Actions are used in menus and keyboard/mouse bindings.
 	Use the automatic placement policy to move the active window to a
 	position on its output that will minimize overlap with other windows.
 
+*<action name="Shade" />*++
+*<action name="Unshade" />*++
+*<action name="ToggleShade" />*
+	Set, unset, or toggle, respectively, the "shaded" state of the active
+	window. When shaded, window contents are hidden, leaving only the
+	titlebar visible. Full-screen windows or those without server-side
+	decorations (including those for which the server-side titlebar has been
+	hidden) are not eligible for shading.
+
 *<action name="None" />*
 	If used as the only action for a binding: clear an earlier defined
 	binding.

docs/menu.xml

@@ -12,6 +12,9 @@
   <item label="Fullscreen">
     <action name="ToggleFullscreen" />
   </item>
+  <item label="Roll up/down">
+    <action name="ToggleShade" />
+  </item>
   <item label="Decorations">
     <action name="ToggleDecorations" />
   </item>

docs/rc.xml.all

@@ -331,6 +331,14 @@
         <action name="Focus" />
         <action name="Raise" />
       </mousebind>
+      <mousebind direction="Up" action="Scroll">
+        <action name="Unshade" />
+        <action name="Focus" />
+      </mousebind>
+      <mousebind direction="Down" action="Scroll">
+        <action name="Unfocus" />
+        <action name="Shade" />
+      </mousebind>
     </context>
 
     <context name="Title">