Allow idle time higher than 59:59 minutes

[j8r]

Currently on LXQt 1.4.0, the Idle section, AC Idle time and Battery Idle time fields are limited to 59:59 minutes at most.

People may want to have more than 1 hour before suspending (or other action).

Adding one more digit will allow to set a time up to 16 hours, but we may want to add 2 digits.

[stefonarch]

I always wondered if we really need seconds there too.