tree-wide: use enum types/constants where appropriate

- add LAB_WINDOW_TYPE_INVALID in place of literal -1
- document more clearly that enum lab_view_criteria is a bitset
- other one-off replacements of integer values/types for consistency

Note: variables of type enum lab_view_criteria are already used
extensively throughout the code to contain combinations of the declared
enum values. I am not introducing any new usage here, just changing the
single uint32_t to be consistent with all the other usages.

include/osd.h

@@ -43,7 +43,6 @@ struct window_switcher_field {
 struct buf;
 struct view;
 struct server;
-enum lab_cycle_dir;
 
 /* Begin window switcher */
 void osd_begin(struct server *server, enum lab_cycle_dir direction);