refactor(sign): move legacy signs to extmarks

Problem:  The legacy signlist data structures and associated functions are
          redundant since the introduction of extmark signs.
Solution: Store signs defined through the legacy commands in a hashmap, placed
          signs in the extmark tree. Replace signlist associated functions.

Usage of the legacy sign commands should yield no change in behavior with the
exception of:
  - "orphaned signs" are now always removed when the line it is placed on is
    deleted. This used to depend on the value of 'signcolumn'.
  - It is no longer possible to place multiple signs with the same identifier
    in a single group on multiple lines. This will now move the sign instead.

Moreover, both signs placed through the legacy sign commands and through
|nvim_buf_set_extmark()|:
  - Will show up in both |sign-place| and |nvim_buf_get_extmarks()|.
  - Are displayed by increasing sign identifier, left to right.
    Extmark signs used to be ordered decreasingly as opposed to legacy signs.

1 files changed, 4 insertions, 3 deletions

diff --git a/src/nvim/extmark.h b/src/nvim/extmark.h
index 771edf3983..b4979c4aea 100644
--- a/src/nvim/extmark.h
+++ b/src/nvim/extmark.h
@@ -84,9 +84,10 @@ typedef enum {
 typedef enum {
   kExtmarkNone = 0x1,
   kExtmarkSign = 0x2,
-  kExtmarkVirtText = 0x4,
-  kExtmarkVirtLines = 0x8,
-  kExtmarkHighlight = 0x10,
+  kExtmarkSignHL = 0x4,
+  kExtmarkVirtText = 0x8,
+  kExtmarkVirtLines = 0x10,
+  kExtmarkHighlight = 0x20,
 } ExtmarkType;
 
 // TODO(bfredl): reduce the number of undo action types