+15 feature doesn't work on New Incidents

[jdalbey]

+15 feature (r84) seems to work on loaded incidents but not new ones.
Steps to reproduce:
Open the app, choose Incidents > New Incident.
Complete the dialog, use incident length of 15.
In the incident editor window that appears, add an event at 13:00.
Observe the timeline appear.
Click the +15:00 button. There is no effect.

[bmcguffin]

The threshold value used to determine whether or not the timeline tick panel should be enlarged was improperly calculated at small sizes. Thus, using the +15:00 button when the whole window was less that 15:00 wide resulted in a bad value. The calculation was corrected and now gives the correct value.