code cleanup and todos review

including removing of dead (unused + not working) tree model classes
TargetsTreeModel, TasksTreeModel and TreeModelWithStructur, plus trivial
(almost useless) model class TextSetModel
ui: adding last execution duration on task detail and alerts pages

this is related to a new lastTotalMillis attribute of Task class
ui: webconsole.cssoverload make possible to customize css

thus it is possible to add some minor ui changes (such as adding a logo)
using only qron.conf configuration

events1.conf uses it to display a work in progress logo background

also imported (with minor changes) two work in progress logos from
openclipart.org