Graph editor nicer UI

[kidzik]

Graph editor looks nice, but it needs some UI improvements:

• the texts is outside the box
• the background is very dark and sad
• it would be cool to have arrows
• etc. (let's discuss)

[thibaultcalvayrac]

In 9fe346d79d582da3bd2e12ece3a6bc51ac86147c, we added a context menu to edit and connect activities. Here is a recap of our GUI modifications:

• [x] Text fits nicely in box
• [x] Text is bigger
• [x] Pick nice colors
• [x] Add arrows to connections
• [x] Erase button on activities
• [x] Context menu to edit and connect activities
• [x] Erase button on connections
• [x] Inspect button on activities (display infos -stats- below graph)
• [x] Inspect button on connections (display infos -operator type- below graph)
• [x] Clear graph option in contextual menu
• [x] Filter activities in the activity selector
• [ ] Color palette on activities
• [ ] Size of activities
• [x] Confirmation modal when deleting or saving scenario
• [x] Confirmation modal when editing scenario (suggest duplication if students are already enrolled)
• [x] Duplicate button on scenarios