[cuegui] Fix issue with jobs changing order (bouncing) when ordered on the JobMonitor plugin

[DiegoTavares]

Issue being fixed.
When values on a columns being used for sorting repeat, jobs with similar values on the given column might change order (bounce) when the job data for other columns updates.

Summarize your change.
This fix proposed and implemented by @Samer-sef appends the job start_time to the columns sorting key to ensure sorting keys are different enough to prevent bouncing.

[linux-foundation-easycla]

• ✅ login: DiegoTavares / name: Diego Tavares (42b687f)
• ❌ The email address for the commit (d68c083) is not linked to the GitHub account, preventing the EasyCLA check. Consult this Help Article and GitHub Help to resolve. (To view the commit's email address, add .patch at the end of this PR page's URL.) For further assistance with EasyCLA, please submit a support request ticket.

[ramonfigueiredo]

@Samer-sef's contribution to this PR happened while he was still at SPI. Even though he has signed the CLA, the check will fail as the email assigned to this commit is no longer associated with his GitHub account.

[ramonfigueiredo]

@DiegoTavares

Approved!