Added tooltip for expand/collapse samplers button.

[Ashnidh]

Resolves #12998.
I added tooltip for the expand/collapse samplers buttons.
The tooltip used is as follows:

Expand/Collapse Samplers
Toggle expanded samplers view.

I have tested the same for all the skins available.
Kindly review.

Here are the screenshots taken in different skins -

[ronso0]

Thank you for this PR.
Please sign the Mixxx Contributor Agreement and comment here when you have done so. It gives us permission to distribute your contribution under the GPL v2 or later license and the Apple Mac App Store. It is also helpful for us to have contact information for contributors in case we may need it in the future.

Please comment here when done.

[Ashnidh]

Thank you sir. I have signed the agreement.

[ronso0]

Thanks.
LGTM

Congratulations for your first merged PR!
(waiting for CI)

[daschuer]

Hi @Ashnidh, I like to add your to the contributor list in the Mixxx about box. Should I write your full name or just Ashnidh?

[Ashnidh]

Hello @daschuer.
You many mention my full name - Ashnidh Khandelwal
Thank you.