Add ESCET logo to ESCET IDE launchers for Linux and macOS

We added it for the Windows launcher in #791 (closed).

Next steps:

• For Linux, we need an XPM icon. Add the icon to the misc/escet-logo folder and to the product.
• For macOS, we need an ICNS icon. Add the icon to the misc/escet-logo folder and to the product.

Both should contain the ESCET logo in various resolutions.

See also:

• !840 (comment 1986938) for how it looks on Windows.
• !840 (comment 1986935) for the formats the Windows icon contains.
• !840 (comment 1986942) for some ideas on which tools to use to make the icons for Linux/macOS.