Make changes to programslots more robust

[equinox0815]

For now the only supported change to programslots is to define a later end dates. Any other case is either prohibited by the interface or does not what the user would expect.
The reason for this is that any other change would most probably mean that at least some timeslots needs to be deleted and possible recreated. Any notes added to those timeslots would be lost. In order to solve this pickle i propose the following:

• after clicking 'save' in the edit show dialog the system should display any changes to timeslots and possible notes that must be made in order to fulfil the change request by the user.
• this dialog should also offer possible conflict resolutions in case the newly generate timeslots overlap with existing timelsots from other programslots.
• the user can then select a conflict resolution and confirm the transaction or click 'abort'

[eigenwijsje]

@equinox0815: Challenge accepted!