nemo-action-bar: add configurable action bar

[ClaudiuSchuster]

This adds Nemo Action Bar 2.0.0, an independent OSS Singularity Nemo
extension that places configurable native GTK actions directly above Nemo's
directory view.

Nemo Action Bar screenshot — default action bar in Nemo

Maintainer: @ClaudiuSchuster.
Package: nemo-action-bar.

The extension exposes an allowlisted set of Nemo-native actions through a
compact configurable action bar. It supports JSON configuration for button
order, separators, labels, icons, shortcuts and live reload.

Copy Paths copies selected file/folder paths as newline-separated text, or
copies the current local folder path when nothing is selected. Nemo's own
actions and dialogs handle file operations; the JSON configuration does not
execute arbitrary commands.

Requires Nemo with nemo-python support, GTK 3/GObject Introspection bindings
and Python 3. The contribution uses the existing per-project Debian layout of
this repository and is GPL-2.0-or-later.

Only the nemo-action-bar/ project directory plus the required build-order
and Readme.md entries are added. The social preview and screenshot above are
pinned to the public v2.0.0 source commit.

Validation

• python3 -m py_compile and JSON validation pass for the staged sources.
• dpkg-buildpackage -us -uc -b passes in Ubuntu 24.04.
• Lintian reports no findings for the resulting package.
• bash -n build and shellcheck build pass.
• The packaged payload contains only the Nemo extension, configuration, icon
  and Debian documentation; no Python egg-info payload is installed.
• Live Linux Mint/X11 validation with Nemo 6.6.3 covers the current-folder
  fallback with no selection and selected-folder path copying.
• The project PR #2
  passed its protected checks and was squash-merged into main as
  56f0f5f1.
• The public v2.0.0 GitHub Release
  is published from that exact merge commit and includes the Debian package.

Project source and v2.0.0 changes
· v2.0.0 GitHub Release