Add awful.client.property.persist()

diff --git a/lib/awful/client.lua.in b/lib/awful/client.lua.in
index f72e9be..fa4ac1f 100644
--- a/lib/awful/client.lua.in
+++ b/lib/awful/client.lua.in
@@ -18,6 +18,7 @@ local capi =
     client = client,
     mouse = mouse,
     screen = screen,
+    awesome = awesome,
 }
 
 -- we use require("awful.screen") inside functions to prevent circular dependencies.
@@ -34,6 +35,8 @@ client.data.focus = {}
 client.data.urgent = {}
 client.data.marked = {}
 client.data.properties = setmetatable({}, { __mode = 'k' })
+client.data.persistent_properties_registered = {} -- keys are names of persistent properties, value always true
+client.data.persistent_properties_loaded = setmetatable({}, { __mode = 'k' }) -- keys are clients, value always true
 
 -- Functions
 client.urgent = {}
@@ -868,6 +871,12 @@ end
 -- @param prop The property name.
 -- @return The property.
 function client.property.get(c, prop)
+    if not client.data.persistent_properties_loaded[c] then
+        client.data.persistent_properties_loaded[c] = true
+        for p in pairs(client.data.persistent_properties_registered) do
+            client.property.set(c, p, c:get_xproperty("awful.client.property." .. prop))
+        end
+    end
     if client.data.properties[c] then
         return client.data.properties[c][prop]
     end
@@ -882,10 +891,26 @@ function client.property.set(c, prop, value)
     if not client.data.properties[c] then
         client.data.properties[c] = {}
     end
+    if client.data.persistent_properties_registered[prop] then
+        c:set_xproperty("awful.client.property." .. prop, value)
+    end
     client.data.properties[c][prop] = value
     c:emit_signal("property::" .. prop)
 end
 
+function client.property.persist(c, prop, type)
+    local prop = "awful.client.property." .. prop
+    capi.awesome.register_xproperty(prop, type)
+    client.data.persistent_properties_registered[prop] = true
+
+    -- Make already-set properties persistent
+    for c, tab in pairs(client.data.properties) do
+        for p, val in pairs(tab) do
+            c:set_xproperty(prop, val)
+        end
+    end
+end
+
 ---
 -- Returns an iterator to cycle through, starting from the client in focus or
 -- the given index, all clients that match a given criteria.
@@ -960,6 +985,9 @@ capi.client.connect_signal("unmanage", client.urgent.delete)
 
 capi.client.connect_signal("unmanage", client.floating.delete)
 
+-- Register persistent properties
+client.property.persist(c, "floating", "boolean")
+
 return client
 
 -- vim: filetype=lua:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:textwidth=80