Add scan button, sort dropdown, and improved empty state to library view

Add a scan button and sort dropdown (Name A-Z, Recently Added, Size)
to the library header bar. Change the empty state to friendlier text
with Scan and Browse Catalog buttons. Default view mode changed to
list. Sort preference persisted via GSettings.

data/app.driftwood.Driftwood.gschema.xml

@@ -26,10 +26,20 @@
         <choice value='grid'/>
         <choice value='list'/>
       </choices>
-      <default>'grid'</default>
+      <default>'list'</default>
       <summary>Library view mode</summary>
       <description>The library view mode: grid or list.</description>
     </key>
+    <key name="sort-mode" type="s">
+      <choices>
+        <choice value='name'/>
+        <choice value='recently-added'/>
+        <choice value='size'/>
+      </choices>
+      <default>'name'</default>
+      <summary>Library sort mode</summary>
+      <description>How to sort the library: name, recently-added, or size.</description>
+    </key>
     <key name="color-scheme" type="s">
       <choices>
         <choice value='default'/>