Fix map layer actions rendering in attribute table

[3nids]

• for a single feature, only Layer and SingleFeature actions are now listed, not MultipleFeatures anymore (showing Layer is debatable)
• if a selection exists and the context menu is shown over a selected row, MultipleFeatures are listed in a new section (see screenshot)

[qgis-bot]

The backport to release-3_10 failed:

The process 'git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-release-3_10 release-3_10
# Navigate to the new working tree
cd .worktrees/backport-release-3_10
# Create a new branch
git switch --create backport-34266-to-release-3_10
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 4fb42ae68dde644d450e8527110b7b50004195bc
# Push it to GitHub
git push --set-upstream origin backport-34266-to-release-3_10
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-3_10

Then, create a pull request where the base branch is release-3_10 and the compare/head branch is backport-34266-to-release-3_10.