Add a button to open the settings

If you already have kdeconnect on your phone, the call to action to install
it doesn't make sense.

This doesn't add a string because it reuses one that was already in the
plasmoid
2 jobs for master in 7 minutes and 31 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #17142
linux kf5-qt5 qt5.12

00:07:16

34.0%
passed #17143
windows
windows kf5-qt5 qt5.13

00:07:31