modernize UI

proposals:
1. instead of using a QMessageBox to indicate e.g. too few lines selected use iface.messageBar (with duration), so user needs not click on _OK_ before being able to continue
2. or (better) activate/deactivate tools according to a proper selection condition
3. simplify the tool bar by removing redundancy, try to merge all _select something_ into one tool (i.e. seperate tools for line, point, vertex). Proposal 2 needs to be in place for this. E.g. select with left click, deselect with right click and keep the order in which the elements were selected. Thus if two elements are needed by a tool the last two will be used for processing, no matter if more have been selected.