MultiMCGH-4966 Refactor the desktop entry

Changed the desktop file icon lookup, to enable app icon theming.
The Icon line uses the icon name instead of the full path.
For this to work, the icon is moved to a directory specified by the
standard (`/usr/share/icons/hicolor/scalable/apps/`).
As the standard highly suggests, a 48x48 png icon is also added
(rendered from the svg with inkscape).

Added the `StartupWMClass` directive, so that window managers can
associate the created windows with the desktop icon.

Changed the rpm spec file to reflect the aforementioned changes.

Fixes MultiMC#4966

launcher/package/rpm/MultiMC5.spec

@@ -26,8 +26,11 @@ A local install wrapper for MultiMC
 
 %install
 mkdir -p %{buildroot}/opt/multimc
-install -m 0644 ../ubuntu/multimc/opt/multimc/icon.svg %{buildroot}/opt/multimc/icon.svg
 install -m 0755 ../ubuntu/multimc/opt/multimc/run.sh %{buildroot}/opt/multimc/run.sh
+mkdir -p %{buildroot}/%{_datadir}/icons/hicolor/48x48/apps
+install -m 0644 ../ubuntu/multimc/usr/share/icons/hicolor/48x48/apps/multimc.png %{buildroot}/%{_datadir}/icons/hicolor/48x48/apps/multimc.png
+mkdir -p %{buildroot}/%{_datadir}/icons/hicolor/scalable/apps
+install -m 0644 ../ubuntu/multimc/usr/share/icons/hicolor/scalable/apps/multimc.svg %{buildroot}/%{_datadir}/icons/hicolor/scalable/apps/multimc.svg
 mkdir -p %{buildroot}/%{_datadir}/applications
 desktop-file-install --dir=%{buildroot}%{_datadir}/applications ../ubuntu/multimc/usr/share/applications/multimc.desktop
 
@@ -41,8 +44,11 @@ appstream-util validate-relax --nonet %{buildroot}%{_metainfodir}/multimc.metain
 
 %files
 %dir /opt/multimc
-/opt/multimc/icon.svg
 /opt/multimc/run.sh
+%dir %{_datadir}/icons/hicolor/48x48/apps
+%{_datadir}/icons/hicolor/48x48/apps/multimc.png
+%dir %{_datadir}/icons/hicolor/scalable/apps
+%{_datadir}/icons/hicolor/scalable/apps/multimc.svg
 %{_datadir}/applications/multimc.desktop
 %{_metainfodir}/multimc.metainfo.xml
 %dir %{_mandir}/man1

launcher/package/ubuntu/multimc/usr/share/applications/multimc.desktop

@@ -1,12 +1,13 @@
 [Desktop Entry]
 Categories=Game;
 Exec=/opt/multimc/run.sh
-Icon=/opt/multimc/icon.svg
+Icon=multimc
 Keywords=game;Minecraft;
 MimeType=
 Name=MultiMC 5
 Path=
 StartupNotify=true
+StartupWMClass=org.multimc.MultiMC
 Terminal=false
 TerminalOptions=
 Type=Application