This fixes #265 (closed). Instead of destroying the contacts_layer actor in on_show, all markers are removed from contacts_layer (results in clean layer to start with).

• Refactor checks for the champlain and clutter libraries
• Remove layers.py (obsolete since removal of osmgpsmap)
• Put scale in upper left corner of the map (not on top of map info anymore)
• Make own marker translatable
• Add go_to own marker on preset change and center_view on show (user's location not out of sight when changes happen)
• Add default text to InputDialog input box