The appointment/meeting editor is somewhat unclear for users who don't understand the way calendars work.

Steps to Reproduce:
1. Subscribe to a webcal
2. Select the webcal
3. Attempt to create a new appointment
  
Actual results:
Can't type anything; insensitive except for the calendar selection area.  It's not clear why things are insensitive.

Version:
evolution 2.5.3

Suggested fix:
Add a descriptive hint below the Calendar combobox with an "information" stock icon and text along the line of the following strings:

for a new meeting/appointment:
------------------------------
if you don't have permission to edit the calendar:
"You do not have permission to edit the "NAME_OF_CALENDAR" calendar.  You will need to select a different calendar to be able to add this [appointment|meeting]"

if it's a subscribed webcal:
"You cannot edit web calendars directly.  You will need to select a different calendar to be able to add this [appointment|meeting]"

etc

This hint would become disappear/change as different calendar are selected.

when editing a pre-existing meeting/appointment:
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I'm not yet sure exactly what the behaviour here ought to be