[#2279] Hooked up the Billable filter to adjust the Timesheet's Activities.

* Hooked into the TimesheetController to modify the Timesheet activities
* Added TimeEntryActivity#find_with_billable_values to find all the TimeEntryActivities
  which have the value in the billable custom field.

lib/overhead_timesheet_hook.rb

@@ -41,4 +41,11 @@ class OverheadTimesheetHook < Redmine::Hook::ViewListener
                                 :selected_values => selected_values
                               })
   end
+
+  def plugin_timesheet_controller_report_pre_fetch_time_entries(context = {})
+    if context[:params] && context[:params][:timesheet] && context[:params][:timesheet][:billable]
+      activities = TimeEntryActivity.find_with_billable_values(context[:params][:timesheet][:billable])
+      context[:timesheet].activities = activities.collect(&:id) unless activities.empty?
+    end
+  end
 end