Display panel along percent of screen edge

Set the length of the panel according to a specified percent value, so
it can span only part of the screen edge.

Position the panel at the start, middle, or end of the screen edge, as
specified.

Tests okay with multiple monitors.

May also satisfy home-sweet-gnome/dash-to-panel#726

schemas/org.gnome.shell.extensions.dash-to-panel.gschema.xml

@@ -40,6 +40,11 @@
     <value value='2' nick='LEFT'/>
     <value value='3' nick='RIGHT'/>
   </enum>
+  <enum id='org.gnome.shell.extensions.dash-to-panel.anchor'>
+    <value value='0' nick='START'/>
+    <value value='1' nick='MIDDLE'/>
+    <value value='2' nick='END'/>
+  </enum>
   <enum id='org.gnome.shell.extensions.dash-to-panel.proximityBehavior'>
     <value value='0' nick='ALL_WINDOWS'/>
     <value value='1' nick='FOCUSED_WINDOWS'/>
@@ -87,6 +92,16 @@
       <summary>Panel element positions</summary>
       <description>Panel element positions (JSON).</description>
     </key>
+    <key type="i" name="panel-length">
+      <default>100</default>
+      <summary>Percentage of screen edge for panel to span</summary>
+      <description>Set the length of the panel, in percent. Horizontal or vertical, according to panel orientation.</description>
+    </key>
+    <key name="panel-anchor" enum="org.gnome.shell.extensions.dash-to-panel.anchor">
+      <default>'MIDDLE'</default>
+      <summary>Position along screen edge</summary>
+      <description>Where to show the panel if it is not the full length of the screen edge.</description>
+    </key>
     <key type="i" name="panel-size">
       <default>48</default>
       <summary>Panel size</summary>