Please describe the problem:
When editing calendars, it turned out that planner did change the ids of the calendars (when saving the file). That way, the changes found when comparing the current save file with the last save file are unneccessarily large.

Steps to reproduce:
1. Have more than one calendar
2. Modify to of them
3. Save and compare saved files


Actual results:
calendar IDs change

Expected results:
calendar IDs are persistent

Does this happen every time?
not tried multiple times

Other information:
Example (diff -u snipplets):
-      <calendar id="2" name="Person1">
+      <calendar id="2" name="Person2">

         <days>
-          <day date="20061121" type="day-type" id="1"/>
-          <day date="20061122" type="day-type" id="1"/>
-          <day date="20061120" type="day-type" id="1"/>
+          <day date="20061110" type="day-type" id="1"/>
         </days>

...
-      <calendar id="4" name="Person2">
+      <calendar id="4" name="Person1">
         <days>
-          <day date="20061110" type="day-type" id="1"/>
+          <day date="20061121" type="day-type" id="1"/>
+          <day date="20061122" type="day-type" id="1"/>
+          <day date="20061120" type="day-type" id="1"/>
         </days>