UI, image-source: Add slide counter to slideshow toolbar

[cg2121]

Description

This shows the current slide and total number of slides in the slideshow toolbar.

Motivation and Context

Better feedback to users.

How Has This Been Tested?

Created slideshow and made sure the slide count and current slide were updated.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• [x] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [x] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] I have included updates to all appropriate documentation.

[cg2121]

Updated to remove duplicate code.