fix: improve icalendar export

[rros]

Type of change(s)

• Bug fix
• Feature / enhancement
• Infrastructure / tooling (CI, build, deps, tests)
• Documentation

What changed and why

The implementation of the IcsExporter class has been changed to do proper normalizing, escaping and line folding as specified in RFC 5545. I created functions to write content lines using the proper line endings and to do line folding for all content lines, not just descriptions.
Also there is a method added to write TEXT properties, which have additional escaping rules for newlines and characters like ',' and ';'.

Tests performed

I did some tests with long lines, line breaks and characters like , and ; which should be escaped in the output.

For example a to-do with a long title and a long description:
Before

BEGIN:VCALENDAR
PRODID:-//Fossify//NONSGML Event Calendar//EN
VERSION:2.0
BEGIN:VTODO
SUMMARY:This task has a very long title and doesn't fit on a single content line. It must be folded properly
UID:6272ad534681402ca13deee8c9d2a5811771114583194
X-FOSSIFY-CATEGORY-COLOR:-8417344
CATEGORIES:Lokale agenda
LAST-MODIFIED:20260215T234152Z
DTSTART;VALUE=DATE:20260215
DTSTAMP:20260218T181614Z
DESCRIPTION:The description is also way too long for a single content line and must als
 o be folded properly\n\nNewlines should also be properly handled.\n\nAs wel
 l as ";" and ","
BEGIN:VALARM
DESCRIPTION:Herinnering
ACTION:DISPLAY
TRIGGER:-P0DT0H10M0S
END:VALARM
END:VTODO
END:VCALENDAR

After

BEGIN:VCALENDAR
PRODID:-//Fossify//NONSGML Event Calendar//EN
VERSION:2.0
BEGIN:VTODO
SUMMARY:This task has a very long title and doesn't fit on a single content
  line. It must be folded properly
UID:6272ad534681402ca13deee8c9d2a5811771114583194
X-FOSSIFY-CATEGORY-COLOR:-8417344
CATEGORIES:Lokale agenda
LAST-MODIFIED:20260215T234152Z
DTSTART;VALUE=DATE:20260215
DTSTAMP:20260218T095821Z
DESCRIPTION:The description is also way too long for a single content line 
 and must also be folded properly\n\nNewlines should also be properly handl
 ed.\n\nAs well as "\;" and "\,"
BEGIN:VALARM
DESCRIPTION:Herinnering
ACTION:DISPLAY
TRIGGER:-P0DT0H10M0S
END:VALARM
END:VTODO
END:VCALENDAR

I used https://icalendar.org/validator.html to validate the results.

Closes the following issue(s)

• Closes Export file can't be importet to Thunderbird because of wrong escape sequences #899

Checklist

• I read the contribution guidelines.
• I manually tested my changes on device/emulator (if applicable).
• I updated the "Unreleased" section in CHANGELOG.md (if applicable).
• I have self-reviewed my pull request (no typos, formatting errors, etc.).
• I understand every change in this pull request.