Implement new criterion 'workspace'.

If the match expression is a plain number (e.g., '99'), the number of a workspace will be compared strictly. Otherwise, the match expression is taken as a regular expression and compared against the workspace's name.
This allows all of the following:

for_window [workspace=5] ...
for_window [workspace="5:foo"] ...
for_window [workspace="foo"] ...

fixes #1769

parser-specs/commands.spec

@@ -51,6 +51,7 @@ state CRITERIA:
   ctype = 'con_mark'    -> CRITERION
   ctype = 'title'       -> CRITERION
   ctype = 'urgent'      -> CRITERION
+  ctype = 'workspace'   -> CRITERION
   ']' -> call cmd_criteria_match_windows(); INITIAL
 
 state CRITERION:

parser-specs/config.spec

@@ -170,6 +170,7 @@ state CRITERIA:
   ctype = 'con_mark'    -> CRITERION
   ctype = 'title'       -> CRITERION
   ctype = 'urgent'      -> CRITERION
+  ctype = 'workspace'   -> CRITERION
   ']'
       -> call cfg_criteria_pop_state()