frontend: verify that executables exists in path

references #114
frontend: handle when it fails to generate preview

references #114
frontend: handle when failing to get list of transitions

fixes #114
Merge commit 'aae1c81'
frontend: use string instead of select for display

purely theoretically one could use network-based connection or another display-server than the one the frontend runs in. In addition the long-time goal is to move this setting away from frontend to backend instance (i.e. support multiple instances)
frontend: replace xorg_query with python-xlib
frontend: handle when it fails to connect to X
Merge branch 'drop_xorg_query'