Improved Clipboard on Linux

[Bamboy]

This PR improves the LinuxClipboard class:

• Fixed xclip parameters to be more compatible with linux desktop environments.
• Added compatibility for working with xsel.
• Added warning in the console if neither xclip or xsel packages are installed on the system, about reduced clipboard function.
• ...and also fixed a minor problem elsewhere in the code to please Rider.

[rds1983]

thanks @Bamboy!