Add timer for `--multiple-targets` and `--multiple-morphs`

[Sparks29032]

Problem

When complicated morphs (e.g. --smear and --smear-pdf) are enabled, the morphing takes a while when many files are involved. A timer would be helpful.

Proposed solution

See title. Let's avoid tqdm or other package imports.

[Sparks29032]

Quality of life change, moved to 0.3.1.

[sbillinge]

moved to 0.4.0