[POST-VIS] Give warning if there is no contact data

[POST-VIS] Give warning if there is no contact data

Problem to solve

First problem

If there is no contact data file contacts.txt, the pushbutton "show_contacts" in PostVisualizationWindow is useless. If the user pushes the button, nothing happens.

Second problem

If you move your cursor onto the pushbutton "show_contacts", it says !ProjectView.btnShowContacts.tooltip! instead of show contacts

Further details

1. Give the user feedback, that his action has no effect since there is no contact data file "contacts.txt" provided.
2. Show show contacts instead of !ProjectView.btnShowContacts.tooltip!