[Feature request] Option to require permission before backing up

Is your feature request related to a problem? Please describe.
Timeshift can take quite a lot of system resource on my machine. It's not always convenient for it to trigger whenever it chooses to do so -- for example, in the middle of an online meeting. Even if it is convenient, I might prefer the chance to save my work and take a short break while the backup is being created.

Describe the solution you'd like
I would like an option to set such that it will request permission to run before taking a backup, with the alternatives being to proceed immediately, or to come back and ask again in five minutes or a couple of hours, say (perhaps allowing for configurable intervals). This would (a) would give me a little notice to stop doing any heavy activity, save work etc, and (b) allow me to pick a more suitable time if now really isn't good.

Describe alternatives you've considered
This would address #122, but by a different -- and arguably better -- means.

Start by tracing Timeshift’s existing scheduled-snapshot path and how it invokes backups. Compare that flow with the requested permission prompt, proceed choice, delayed retry, and configurable intervals; done means the option works as requested while preserving current behavior when disabled.