Add labwc to XDG_CURRENT_DESKTOP to support a portals.conf (#1716)

Add labwc to XDG_CURRENT_DESKTOP to support a portals.conf (#1716)

The previous `UseIn` key was deprecated in xdg-desktop-portal 1.17/1.18.
It has been superceded by the portals.conf structure so that each
desktop can configure the precise desired structure for portals. In
addition, support was added to the Desktop Entry Specifications to
support a `DesktopNames` key that login managers will use to set
XDG_CURRENT_DESKTOP.

* [portals.conf
  Documentation](https://github.com/flatpak/xdg-desktop-portal/blob/main/doc/portals.conf.rst.in)
* [Example
  sway-portals.conf](https://salsa.debian.org/swaywm-team/sway/-/blob/debian/sid/debian/sway-portals.conf)
* [Desktop Entry
  Specifications](https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html)

Ref: flatpak/xdg-desktop-portal#955