Schedule Visualiser - Add an ability to load the schedule from clipboard

The "Open" button should open a modal dialog that asks the user to choose between 2 options. Upload a file or paste JSON directly into an input box.

• Add OpenDialog component
• Show the dialog when pressing "Open"
• Add an "X" to close the dialog
• Add ScheduleUpload to the top part
• Hide the dialog when clicking outside the window
• Add SchedulePaste component
• Add SchedulePaste to the bottom part
• Implement SchedulePaste component
• Connect all components to OpenDialog's callback
• Add unit tests
• Add CSS