Increase/decrease heading level of more than one heading at a time

[ClarusDignus]

Problem

When copying page/post contents to other pages/posts, I sometimes have to manually increase/decrease the heading level of dozens of headings.

E.g., a page's content consists of a h2 heading and h3 headings. I copy and paste this content into a new page under a h3 heading, so for the pasted content, I need to decrease the headings; the h2 heading needs to be decreased to h4, and the h3 elements need to be decreased to h5.

Solution

When multiple blocks are selected/highlighted, there should be:

1. a button to increase all selected/highlighted headings by one heading level.
2. a button to decrease all selected/highlighted headings by one heading level.

Something similar already exists. When multiple blocks are selected/highlighted, up and down arrows appear above the selected/highlighted blocks:

Up/down arrows could similarly be used to increase/decrease the heading levels of multiple headings. A "H" icon could be placed before the up/down arrows to distinguish them from the up/down arrows for moving blocks.

[ellatrix]

Related: allowing non consecutive selection: #16811