In the Agenda mode, disable prev/next buttons when the end of events is reached

[acotiuga]

Steps to reproduce:

1. Create a new calendar
2. Create some event/s in the new calendar
3. From the Calendar navigation bar, select Agenda mode
4. Using < (prev) and > (next) buttons navigate to the old events.
   

Expected result:
When the earliest/latest event in the calendar is reached, the corresponding button should have a disabled aspect
Actual result:
You can click as many times as you want, but the agenda view remains on the earliest/latest events without going beyond.