modernize UI

[bstroebl]

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.