add "Selection" toolbar to the QGIS main window

[alexbruy]

Description

Currently "Attributes" toolbar contains also selection tools. While for tool like "Select by value" it is ok to be in the the "Attributes" toolbar, for spatial selection tools this is a bit strange location. So new "Selection" toolbar added and all selection tools (both spatial and attribute) was moved to this new toolbar.

In future some Processing tools like "Select by Location" can be added to this toolbar too (as asked in #20350).

Fixes #21045.

[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!

[nyalldawson]

@nirvn what do you think? I'm +0

[nirvn]

I like the idea, I'm +1. This attribute toolbar is crowded, and a bit of a potpourri, this PR helps a bit.

[qgis-bot]

@alexbruy
A documentation ticket has been opened at qgis/QGIS-Documentation#5484
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!