Add support for sending environment variables to launched apps

[carterisonline]

Summary

I was having some issues with settings a qt theme using qt5ct, so I implemented an option in the config to send environment variables to apps using GIO's ApplicationFlags.

Notable Changes

• Added a config option called send_environment which controls whether Sirula sends environment variables
• Moved the config variable to the application entry point main
• The app_startup function now takes (after cloning) ownership over the config variable as a parameter instead of loading it inside the function itself.

[DorianRudolph]

Could you tell me what this actually does? I did not understand the documentation. Isn't the environment passed automatically? When I launch a terminal with sirula, the output of env looks the same with or without that flag.