Add description to all events

[gmmars]

It should be clear for any reader when the corresponding event occur. For example, FileUploadFinishedEvent io.jmix.flowui.kit.component.upload.event.FileUploadFinishedEvent is fired when FinishedEvent of Upload is occurred. The description repeats the “question”. It is not clear from the description at what conditions the particular event occurs.

Vaadin provides more detailed description - “FinishedEvent is sent when the upload receives a file, regardless of whether the reception was successful or failed”.

It is very important to provide Jmix users with detailed description without forcing them to search for the information on external resources.