Allow adding Processing tools to Favorites

[alexbruy]

Description

Allow to add/remove Processing algorithms to Favorites. Contrary to the "Recently used" list, Favorites list is populated manually by the user and remains static no matter which algorithms were executed.

To add an algorithm to Favorites use context menu

The context menu is used to remove algorithm from Favorites. This works with the item in Favorites group and with algorithm item itself.

Fixes #29220.

[qgis-bot]

@alexbruy
This pull request has been tagged as requiring documentation.

A documentation ticket will be opened at https://github.com/qgis/QGIS-Documentation when this PR is merged.

Please update the description (not the comments) with helpful description and screenshot to help the work from documentors.
Also, any commit having [needs-doc] or [Needs Documentation] in will see its message pushed to the issue, so please be as verbose as you can.

Thank you!

[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit 01c1c17)

[qgis-bot]

@alexbruy
A documentation ticket has been opened at qgis/QGIS-Documentation#9158
It is your responsibility to visit this ticket and add as much detail as possible for the documentation team to correctly document this change.
Thank you!

[qgis-bot]

@alexbruy

This pull request has been tagged for the changelog.

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