awful.rules: Implement callbacks for individual properties

When a property is now set to a function, the function's return value will be
used for the value of the property. The function gets the new client as its only
argument.

There is no property which accepts a function as its value and thus this change
can't break anything (yeah, famous last words...).

This should fix half of FS#1011. Panels now don't get focused by awful.rules.

Signed-off-by: Uli Schlachter <psychon@znc.in>

awesomerc.lua.in

@@ -334,7 +334,7 @@ awful.rules.rules = {
     { rule = { },
       properties = { border_width = beautiful.border_width,
                      border_color = beautiful.border_normal,
-                     focus = true,
+                     focus = awful.client.focus.filter,
                      keys = clientkeys,
                      buttons = clientbuttons } },
     { rule = { class = "MPlayer" },

lib/awful/rules.lua.in

@@ -65,6 +65,8 @@ local rules = {}
 -- <p>If a client matches multiple rules, their applied in the order they are
 -- put in this global rules table. If the value of a rule is a string, then the
 -- match function is used to determine if the client matches the rule.</p>
+-- <p>If the value of a property is a function, that function gets called and
+-- function's return value is used for the property.</p>
 --
 -- <p> To match multiple clients to a rule one need to use slightly different 
 -- syntax:
@@ -165,6 +167,9 @@ function rules.apply(c)
     end
 
     for property, value in pairs(props) do
+        if property ~= "focus" and type(value) == "function" then
+            value = value(c)
+        end
         if property == "floating" then
             aclient.floating.set(c, value)
         elseif property == "tag" then
@@ -198,7 +203,7 @@ function rules.apply(c)
 
     -- Do this at last so we do not erase things done by the focus
     -- signal.
-    if props.focus then
+    if props.focus and (type(props.focus) ~= "function" or props.focus(c)) then
         client.focus = c
     end
 end