event-editor: Improve event name validation handling

• Fix a bug where clicking the "Add Event" button created an empty title in the event editor dialog.
• Add "error" class when the user writes an invalid event name.
• Add new function to validate event name.

Because the sections are split into files, the event editor will need a signal to check that each section is correct, and each section will have to emit the signal. This will need to be done in a future commit.