Desktop: Add + to access 'Zoom In', allows to use the numpad (#2630)

* add + to access 'Zoom In', allows to use the numpad

This change aligns Joplin's behavior with all other apps that have shortcuts for `Zoom In`.
It therefore improves the user experience.

/ref

https://discourse.joplinapp.org/t/please-help-test-the-new-desktop-pre-release/6445/2?u=tessus
https://github.com/laurent22/joplin/pull/2165#issuecomment-593106648

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