This should ensure that all editors which support
https://editorconfig.org/ use the correct indentation
with spaces (not tabs) by default.

This is hardly a full specification of our coding style,
but it's a correct subset and better than nothing.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/353>
(cherry picked from commit 0ca156a2)

We already include an .editorconfig that is supported by many
editors, including emacs, so no need to repeat an emacs-specific
modeline in every source file.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/353>
(cherry picked from commit 253ddb86)

Meson strongly recommends to use SPDX identifiers for the license
string, and there's no reason for us to not do so.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/353>
(cherry picked from commit 6df49052)

Those are a bit more concise than ''.format(), and have been
in meson for a long time.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/353>
(cherry picked from commit 85ee8829)

This is meson's default formatting, and matches what we already
do in JS.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/353>
(cherry picked from commit 720933b0)

We are supposed to use a 2-space indent, but some inconsistencies
sneaked in over time.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/353>
(cherry picked from commit b9036180)

Either have all arguments on the same line, or have a separate
line for every argument (that is, don't special-case the first
arg).

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/353>
(cherry picked from commit 265935e1)

The script was updated to not require appstream-util to check for
a corresponding release element in metainfo.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/354>
(cherry picked from commit 1218aee8)

PopupImageMenuItems used to position the icon after the label,
so we ended up with our own icon+label items.

However the icon position was changed years ago in the shell, so
inherit from PopupImageMenuItem instead.

This does not only simplify the code a bit, but also pulls in
features we are currently missing, like a11y labelling.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/issues/542
(cherry picked from commit 568826e4)

Loosely based on the README entry, so users know how to actually use the
extension.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/issues/541
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/358>

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