Add generic templates (#307)

- Add generic full-page templates.  (This feature from 3.4 was missing)
- Also add generic half-page, quarter-page, and envelope templates.
- Add several more common envelope sizes to page-sizes.xml
- Qt print backend does not need pwg_size.
    - Replace with pwg_class, to distinguish between ISO and NA sizes

translations/glabels_C.ts

@@ -271,6 +271,33 @@
         <translation type="unfinished"></translation>
     </message>
 </context>
+<context>
+    <name>GenericTemplate</name>
+    <message>
+        <source>Generic</source>
+        <translation type="unfinished"></translation>
+    </message>
+    <message>
+        <source>%1 full-page labels</source>
+        <extracomment>%1 = page size. (e.g. A4)</extracomment>
+        <translation type="unfinished"></translation>
+    </message>
+    <message>
+        <source>%1 envelope</source>
+        <extracomment>%1 = envelope size. (e.g. DL)</extracomment>
+        <translation type="unfinished"></translation>
+    </message>
+    <message>
+        <source>%1 half-page labels</source>
+        <extracomment>%1 = page size. (e.g. A4)</extracomment>
+        <translation type="unfinished"></translation>
+    </message>
+    <message>
+        <source>%1 quarter-page labels</source>
+        <extracomment>%1 = page size. (e.g. A4)</extracomment>
+        <translation type="unfinished"></translation>
+    </message>
+</context>
 <context>
     <name>MergeView</name>
     <message>