Ship desktop icon with systemd-ui

[AsciiWolf]

The applications-system icon currently used by the desktop file is from an adwaita-icon-theme(-legacy). This creates an unnecessary dependency.

This PR bundles the applications-system.svg icon from legacy GNOME/Adwaita icon theme (archived here).

Fixes #9.

[AsciiWolf]

@mathstuf Sorry for the mess, I somehow managed to break my Git tree. Everything should be correct now. Please review, thanks!

[AsciiWolf]

@mathstuf I have fully tested this and the previous PR on my Debian Testing system:

# Install missing packages on my Debian Testing system
$ sudo apt install valac libgee-0.8-dev libnotify-dev

# Create the build dir and build systemd-ui
$ mkdir build
$ cd build
$ meson ..
...
$ ninja
...

# Install systemd-ui
$ sudo ninja install
[0/1] Installing files
Installing src/systemadm to /usr/bin
Installing src/systemd-gnome-ask-password-agent to /usr/bin
Installing man/systemadm.1 to /usr/share/man/man1
Installing /home/asciiwolf/systemd-ui/src/io.systemd.systemadm.desktop to /usr/share/applications
Installing /home/asciiwolf/systemd-ui/src/io.systemd.systemadm.svg to /usr/share/icons/hicolor/scalable/apps
Installing /home/asciiwolf/systemd-ui/src/io.systemd.systemadm.metainfo.xml to /usr/share/metainfo
Installing /home/asciiwolf/systemd-ui/src/systemd-gnome-ask-password-agent.rules to /usr/share/polkit-1/rules.d
Installing /home/asciiwolf/systemd-ui/src/systemd-gnome-ask-password-agent.path to /usr/lib/systemd/user
Installing /home/asciiwolf/systemd-ui/src/systemd-gnome-ask-password-agent.service to /usr/lib/systemd/user
Installing /home/asciiwolf/systemd-ui/LICENSE to /usr/share/doc/systemd-ui
Installing /home/asciiwolf/systemd-ui/CODING_STYLE to /usr/share/doc/systemd-ui

$ cat /usr/share/applications/io.systemd.systemadm.desktop
[Desktop Entry]
Name=systemadm
Comment=Manage systemd units
Exec=systemadm
Icon=io.systemd.systemadm
Terminal=false
Type=Application
Categories=System;Settings;

$ file /usr/share/icons/hicolor/scalable/apps/io.systemd.systemadm.svg
/usr/share/icons/hicolor/scalable/apps/io.systemd.systemadm.svg: SVG Scalable Vector Graphics image, ASCII text, with very long lines (3607)

$ cat /usr/share/metainfo/io.systemd.systemadm.metainfo.xml
<?xml version="1.0" encoding="UTF-8"?>
<!-- Copyright 2014 Zbigniew J\u0119drzejewski-Szmek <zbyszek@in.waw.pl> -->
<component type="desktop-application">
  <id>io.systemd.systemadm</id>
  <metadata_license>CC0-1.0</metadata_license>
  <project_license>GPL-2.0-or-later</project_license>
  <name>systemadm</name>
...

Everything appears to be correct.

[AsciiWolf]

Here are some screenshots:

Desktop entry in GNOME:

App metadata (from the metainfo file) in GNOME Software:

Both correct.

The app window itself still has no icon, but that is a different problem not related to this (or the previous) PR.