`xaos.desktop` file enable multiple-thread support

Currently, the /usr/share/applications/xaos.desktop is like below:

[Desktop Entry]
Version=1.0
Type=Application
Name=XaoS
Comment=Interactive fractal zoomer
Exec=xaos
Icon=xaos
Terminal=false
StartupNotify=false
Categories=Application;Graphics;

However the command xaos can only launch xaos in single thread mode. If passing command xaos -threads $(nproc), it would enable the multi-threading support of xaos. Which make this software a lot faster and reactive.

It would be nice to include this option in next release.

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Admin message

`xaos.desktop` file enable multiple-thread support