Identify written contents variable and flag names (#136)

* name item_flags2[3] "has_written_content"

item_flags2[3] is the flag that indicates that a quire or scroll has one or more itemimprovement_writingst improvements on it.  It's used by workshops to determine if an object might meet the [HAS_WRITING_IMPROVEMENT] requirement for reactions like BIND_BOOK.  (There's also a further check to see if there's an actual improvement in the improvements list.)

* name itemimprovement_writingst::anon_1

itemimprovement_writingst::anon_1 contains the writings in a quire, scroll, or book.  The elements are IDs into df.global.world.written_contents.all.  Naming this variable will also fix the inconvenient name collision with the anon_1 in the base class.

df.itemimprovements.xml

@@ -124,7 +124,7 @@
     </class-type>
 
     <class-type type-name='itemimprovement_writingst' inherits-from='itemimprovement'>
-        <stl-vector type-name='int32_t'/>
+        <stl-vector name='contents' type-name='int32_t' ref-target='written_content'/>
     </class-type>
 
     <enum-type type-name='written_content_type' base-type='int32_t'>

df.items.xml

@@ -47,6 +47,7 @@
         <flag-bit name='has_rider' comment='vehicle with a rider'/>
         <flag-bit name='unk1'/>
         <flag-bit name='grown'/>
+        <flag-bit name='has_written_content'/>
     </bitfield-type>
 
     <enum-type type-name='item_magicness_type' base-type='int16_t'>