Check connectivity edge cases

[invisibleroads]

• Delete using delete or backspace in abstract add mode
• Do not show dummy bus on add line click bus
• Check why removing line endpoint sometimes results in two connections
• Check why moving line endpoint sometimes results in taking another line's bus

[invisibleroads]

@zoek1
You marked yourself as owner for edge cases 15, 16, 17, 18 and 26.

Rodrigo and I will be testing edge cases priority 1-4 today at 4pm EST. Would it be possible for you to send a pull request for your edge cases before 4pm EST or before tomorrow?

https://docs.google.com/spreadsheets/d/1eOu8fMNGESR_q5i6ir-4UGcyjWDg0thT_HdMw6MZfZQ/edit#gid=0