Undo: unified undo system w/ linear history

- Use a single undo history for all operations.
- UndoType's are registered and poll the context to check if they
  should be used when performing an undo push.
- Mode switching is used to ensure the state is correct before
  undo data is restored.
- Some undo types accumulate changes (image & text editing)
  others store the state multiple times (with de-duplication).
  This is supported by checking UndoStack.mode `ACCUMULATE` / `STORE`.
- Each undo step stores ID datablocks they use with utilities to help
  manage restoring correct ID's.
  Needed since global undo is now mixed with other modes undo.
- Currently performs each undo step when going up/down history
  Previously this wasn't done, making history fail in some cases.
  This can be optimized to skip some combinations of undo steps.

grease-pencil is an exception which has not been updated
since it integrates undo into the draw-session.

See D3113

source/blender/blenkernel/BKE_main.h

@@ -84,10 +84,12 @@ typedef struct Main {
 	short minversionfile, minsubversionfile;
 	uint64_t build_commit_timestamp; /* commit's timestamp from buildinfo */
 	char build_hash[16];  /* hash from buildinfo */
-	short recovered;	/* indicate the main->name (file) is the recovered one */
+	char recovered;	/* indicate the main->name (file) is the recovered one */
+	/** All current ID's exist in the last memfile undo step. */
+	char is_memfile_undo_written;
 
 	BlendThumbnail *blen_thumb;
-	
+
 	struct Library *curlib;
 	ListBase scene;
 	ListBase library;