Timeshift: Quota support should be optional

[KillWolfVlad]

Quota system have slightly unpleasant known issues, so will be useful to disable it.

timeshift forcibly execute this commands (without any questions):

• # btrfs quota enable <path>
• # btrfs quota rescan <path>

But without quota will not be possible to know snapshots size (or execute this command# btrfs qgroup show)

I think in settings (and first launch wizard) should be a check box (enabled by default)

[teejee2008]

I have never noticed any performance issues with quotas, but I'll add an option to disable it.

[teejee2008]

Added option in v18.4