matcher: Match all sources when none is provided.

Nobody wants to set this parameter. It is necessary because the old
API allowed `awful.rules` to be used with random for random matching.

This stopped "really" working between the 3.4 and 3.5 release because
the code started to accumulate "corner case" fixes aligned with the
client properties. v4.0 added more ordering and v4.3 added external
sources. After this, it is unusable with external objects, but
`gears.matcher` handle this use case very well.

lib/gears/matcher.lua

@@ -215,12 +215,21 @@ end
 --
 -- @param o The object.
 -- @tparam[opt=nil] table rules The rules to check. List with "rule", "rule_any",
---  "except" and "except_any" keys. If no rules are provided, one is selected at
---  random. Unless more rule sources are added, there is only one to begin with.
--- @treturn table The list of matched rules.
+--  "except" and "except_any" keys. If no rules are provided, all rules
+--  registered with a source will be matched.
 -- @method matching_rules
 function matcher:matching_rules(o, rules)
-    rules = rules or select(2, next(self._matching_rules))
+
+    -- Match all sources.
+    if not rules then
+        local ret = {}
+
+        for _, r in pairs(self._matching_rules) do
+            gtable.merge(ret, self:matching_rules(o, r))
+        end
+
+        return ret
+    end
 
     local result = {}