Notifications: Use libnotify

Use libnotify for meshtasticd desktop notifications.

Add "HAS_LIBNOTIFY" macro to guard for builds where libnotify is expected.
This is not included in buildroot/openwrt builds. (no libnotify except when extra repos are added).

Install desktop icon in the correct location on Debian and Fedora packages.
Update dependencies in packaging and dockerfiles.

.devcontainer/Dockerfile

@@ -34,6 +34,7 @@ RUN apt-get update && export DEBIAN_FRONTEND=noninteractive \
     libxcb-xkb-dev \
     libxkbcommon-dev \
     libinput-dev \
+    libnotify-dev \
     && apt-get clean && rm -rf /var/lib/apt/lists/*
 
 RUN pipx install platformio