update dialog: show update version in availability and restart prompt

[ndoschek]

What it does

Closes GH-578

• extract update info once available from electron updater
• include version number in update available and restart messages if present

e.g., update available:

How to test

1. Follow the test preparation steps here: Update dialog: add Cancel button #428 #549 (comment)
2. Test the update and check if the messages show the version to which the application will update

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[jfaltermeier]

Thanks, works for me!