Refactor VCardManager

* Use consistent method names
* Clear string by '.clear()' instead of '= QString()'
* Improve and add method documentation
* Place '&' on the correct side
3 jobs for refactor/vcard-manager in 23 minutes and 50 seconds (queued for 51 minutes and 34 seconds)
Status Job ID Name Coverage
  Test
passed #67750
build

00:17:36

passed #67748
clazy

00:03:10

passed #67749
qmllint

00:03:02