fullscreen wont work with focusable: true on Linux

To reproduce

To clone and run this repository you'll need Git and Node.js (which comes with npm) installed on your computer. From your command line:

# Clone this repository
git clone https://github.com/hovancik/electron-fullscreen-focusable.git
# Go into the repository
cd electron-fullscreen-focusable
# Install dependencies
npm install
# Run the app
npm start