Icon support on linux

[ShiralynDev]

Adds the icon to the window trough SDL as linux does not support .ico loading like windows.
I'm unsure about the get_files_dir function and the paths might need to be changed.

[cesarBLG]

Thanks for the addition! get_files_dir is used to get the correct path for config (user-configurable parameters that don't usually change), data (variable data changed during runtime), and assets (images, icons, sounds, ...)

[cesarBLG]

For automated build and packaging, it would also be useful to set

install(FILES DMI.bmp DESTINATION ${ETCS_ASSET_DIR})

in DMI/CMakeLists.txt so that the file is automatically copied to the destination folder.

I'll check how the unix define works for android, just in case it breaks something, but don't worry about it, I'll do it myself.

Maybe I could add the Android build to the CI to avoid having to manually test it, but that may confuse users, as the Android app also contains the EVC, and thus may not work with SR.

[ShiralynDev]

I'll check how the unix define works for android, just in case it breaks something, but don't worry about it, I'll do it myself.

If I remember correctly android will be defined as unix but SDL will ignore the setting of icon.

[cesarBLG]

Thanks!