popover: Move Emoji and Terminal to ui file

This allows them to be translated via po so we don't have to maintain
the translation list by hand or care about empty translation files.

The only downside is that the "overlay layouts" in OVERLAY_NAMES need to
match the ones in the ui file but since a missing one is a clean crash
when selected there's little potential for subtle breackage. We could
even ensure consistency by comparing the two at run time but they change
only slowly this is not much of an issue (compared to how e.g. new
languages can be added now).

data/popup.ui

@@ -1,7 +1,16 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<!-- Currently only serves translations until we make popover.rs use it -->
 <interface>
-  <menu>
+  <menu id="app-menu">
+    <item>
+      <attribute name="label" translatable="yes">Emoji</attribute>
+      <attribute name="action">layout</attribute>
+      <attribute name="target">emoji</attribute>
+    </item>
+    <item>
+      <attribute name="label" translatable="yes">Terminal</attribute>
+      <attribute name="action">layout</attribute>
+      <attribute name="target">terminal</attribute>
+    </item>
     <section>
       <item>
         <attribute name="label" translatable="yes">Keyboard Settings</attribute>