Implement left and right click in Gallery component and show implicit images in Gallery grid

[hannahblair]

Description

This PR adds improvements to Gallery as part of the component cleanup.

• A user can now click on the left or right hand side of a preview image to scroll between images
• When the number of images imported into the Gallery component is greater than the specified cells, (ie. the explicit cells) the implicit cells (the extra cells created by CSS grid) are now visible as well as the explicit cells.

Closes:
#4585
#4497

Demo code (create a dir /images containing over 4 images to test the hidden images fix):

import pathlib
import gradio as gr

paths = [path.as_posix() for path in sorted(pathlib.Path('images').glob('*'))]
with gr.Blocks() as demo:
    gr.Gallery(value=paths).style(columns=2,
                                  rows=2,
                                  height="400px",
                                  object_fit='contain')
demo.queue().launch()

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All the demos for this PR have been deployed at https://huggingface.co/spaces/gradio-pr-deploys/pr-4995-all-demos

---

You can install the changes in this PR by running:

pip install https://gradio-builds.s3.amazonaws.com/41c83070b01632084e7d29123048a96c1e261407/gradio-3.38.0-py3-none-any.whl

[gradio-pr-bot]

🦄 change detected

This Pull Request includes changes to the following packages.

Package	Version
@gradio/app	minor
@gradio/gallery	minor
gradio	minor

• Maintainers can select this checkbox to manually select packages to update.

With the following changelog entry.

Implement left and right click in Gallery component and show implicit images in Gallery grid

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[freddyaboulton]

Thanks a lot @hannahblair ! Verified both fixes here: https://huggingface.co/spaces/freddyaboulton/gradio-issue-4497-gallery-collapsed