highlight: Added QuickFixLine highlight group

- Links to Search by default

screen.c: Combine CursorLine with QuickFixLine

- HLF_QFL takes priority over HLF_CUL

docs: Updated to mention QuickFixLine
runtime: Added QuickFixLine to nvimHLGroup
tests: QuickFixLine highlight

src/nvim/globals.h

@@ -486,6 +486,7 @@ typedef enum {
   , HLF_CUC         /* 'cursurcolumn' */
   , HLF_CUL         /* 'cursurline' */
   , HLF_MC          /* 'colorcolumn' */
+  , HLF_QFL         // selected quickfix line
   , HLF_COUNT       /* MUST be the last one */
 } hlf_T;
 
@@ -494,7 +495,7 @@ typedef enum {
 #define HL_FLAGS {'8', '~', 'z', 'Z', '@', 'd', 'e', 'i', 'l', 'm', 'M', 'n', \
                   'N', 'r', 's', 'S', 'c', 't', 'v', 'V', 'w', 'W', 'f', 'F', \
                   'A', 'C', 'D', 'T', '-', '>', 'B', 'P', 'R', 'L', '+', '=', \
-                  'x', 'X', '*', '#', '_', '!', '.', 'o'}
+                  'x', 'X', '*', '#', '_', '!', '.', 'o', 'q'}
 
 EXTERN int highlight_attr[HLF_COUNT];       /* Highl. attr for each context. */
 EXTERN int highlight_user[9];                   /* User[1-9] attributes */