Fix issues exists in linux desktop file

Hi @amir1376

This is fix issues exists in linux desktop file, just copy and paste the code below

```
[Desktop Entry]
Name=ABDownloadManager
GenericName=Download Manager
Comment=A Download Manager that speeds up your downloads
Exec=/opt/abdownloadmanager/bin/ABDownloadManager
Icon=/opt/abdownloadmanager/lib/ABDownloadManager.png
Terminal=false
Type=Application
Categories=Network;
StartupNotify=true
StartupWMClass=com.abdownloadmanager.ABDownloadManager
```

**Note**
- The right path for desktop file is: `/usr/share/applications/ABDownloadManager.desktop`
- The right path for app icon is: `/usr/share/icons/hicolor/512x512/apps/ABDownloadManager.png`

when you make sure you set the right paths, update the desktop file to reflect the changes.