Fix memory leak for battery tooltips

Instead of returning a const string, that is fed into strdup, tooltip
functions are now supposed to return allocated strings. This fixes a
memory leak in the battery tooltip.

This is used instead of simply freeing the memory in the battery
tooltip function, since it also avoids a uselesss strdup().

src/taskbar/task.c

@@ -38,10 +38,10 @@
 timeout* urgent_timeout;
 GSList* urgent_list;
 
-const char* task_get_tooltip(void* obj)
+char* task_get_tooltip(void* obj)
 {
 	Task* t = obj;
-	return t->title;
+	return strdup(t->title);
 }