Add method for getting the download speed

src/backend/transaction_state.rs: Add get_download_speed() impl to TransactionState.

Following the mock-up at https://gitlab.gnome.org/Teams/Design/software-mockups/-/blob/master/adaptive/app-page-install-flow.png.

As explained in the TODO note, there is a bug in which sometimes, the percentage will start at 1.0 and the text Installing... can be seen very briefly.