Display the current version of the application in the Software updates tab of the Settings window

[Korb]

Describe the bug
To manually check for updates, it is convenient to open the main page of the application in Github and the About screen of the application. Workrave does not have the usual main window, but it does have a Settings window, the last item in the navigation panel on the left gives reason to assume that the user will find at least the version of the installed application there. And maybe even the number of the current version, if the application has access to the internet.

To Reproduce
Steps to reproduce the behavior:

1. Open the Workrave Settings window
2. Go to the Software updates tab

Expected behavior
The Software updates tab displays the version of the running Workrave instance.

Screenshots

** Windows (please complete the following information in case you encountered the bug on Windows):

• Windows Version: Windows 10
• Workrave Version 1.10.52

Upd. The context menu of the application icon in the Notification Area has an About item. Perhaps it makes sense to consider the possibility of transferring the update checking functionality to it, as is the case in Mozilla products and in Google Chrome: