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

src/commands.c

@@ -411,6 +411,11 @@ void cmd_criteria_add(I3_CMD, char *ctype, char *cvalue) {
         return;
     }
 
+    if (strcmp(ctype, "workspace") == 0) {
+        current_match->workspace = regex_new(cvalue);
+        return;
+    }
+
     ELOG("Unknown criterion: %s\n", ctype);
 }