Fix window not resizing to fit content, causing clipped Apply dialog

GTK3 toplevel windows on Wayland don't grow to accommodate new content
once mapped, so when heads are (re)populated asynchronously after the
window is shown, or when switching to the "Apply Changes?" header, the
window can keep its stale (too small) size and clip the settings panel
(Apply/Cancel buttons, Transform, Flipped, etc). This matches #5.

- main.c: explicitly resize the window to its natural size after
  rebuilding the head list and after switching the header stack page.
- head.ui: bound the head description label with max-width-chars so a
  long EDID description string can't force the layout wider than
  intended even when wrap/ellipsize are set.

resources/head.ui

@@ -66,6 +66,7 @@
         <property name="wrap">True</property>
         <property name="wrap_mode">word-char</property>
         <property name="ellipsize">end</property>
+        <property name="max_width_chars">40</property>
         <property name="xalign">0</property>
       </object>
       <packing>