patch 8.1.1391: no popup window support

Problem:    No popup window support.
Solution:   Add initial code for popup windows.  Add the 'wincolor' option.
diff --git a/src/feature.h b/src/feature.h
index bfbabe0..c613c4b 100644
--- a/src/feature.h
+++ b/src/feature.h
@@ -477,7 +477,7 @@
 #endif
 
 /*
- * +textprop		Text properties
+ * +textprop		Text properties and popup windows
  */
 #if defined(FEAT_EVAL) && defined(FEAT_SYN_HL)
 # define FEAT_TEXT_PROP