Adding keyboard mnemonic for `Upgrading system...` button on updater window

Hi,

Just noticed that no mnemonic were used for the most important button of the updater window.
I'd love to be able to upgrade my system just by pressing alt+U, like we're currently able to close the window using alt+C.

So I just switched from `gtk_button_new_with_label` to `gtk_button_new_with_mnemonic`.
I've added french translation for this update, but with a huge file diff, I hope I'm doing it right.