T-index label

[will-moore]

See https://trello.com/c/cVYdrcXM/131-no-time-stamp-use-t-index-instead
Adds support for labels showing T-index.
Useful if the image doesn't have timestamp metadata.

To test:

• Add Timelapse images to figure, with and without Time-stamp metadata.
• Choose to create label from Time (index) option (NB: this option will not be disabled in image with no timestamp metadata)
• Label should show the current T-index for the panel
• Export of the figure should also show correct T-index label for both images.

[jburel]

The manipulation of label needs to be reviewed
if for example as add a label from Time (index), in the edit row, I have to manually change the type.
If I click add, a "second" entry is displayed above the first one
This is confusing

[will-moore]

@jburel Not sure I understand your workflow above. This PR doesn't change the existing workflow, it just adds another option.

[jburel]

I understand the change made in this PR
but I think we should review the current implementation

[jburel]

Improvement recorded on the testing sheet.
This can be done in a follow-up PR