Prevent handling onDidChangeActiveTextEditor twice

[Magyarimiki]

The window.onDidChangeActiveTextEditor event is called twice in vscode by design:

• old editor deactivated (event parameter: undefined)
• new editor activated (event parameter: the editor object)

Simply reacting to this event calls the event handler function twice when switching editors.

This PR adds a wrapper to the event handler function to ignore the event for the deactivation and only handle it when the new editor is activated.