ImSequencer: Clicks must start inside button; animate buttons

[idbrii]

• Fix dragging a ImSequencer track start over the remove button and releasing deletes the track.
  • This issue caused a lot of headaches because users didn't understand why they were losing data.
• Fix clicking on ImSequencer duplicate and dragging off to cancel removes the track if you released over the remove button.

Now SequencerAddDelButton behaves like ImGui:Button: requires the click to start inside the button and end inside the button.

When you click a button, expand its size to give feedback that it's pressed.

Bug:

https://user-images.githubusercontent.com/43559/197356263-f66685ff-9da7-4178-bc7d-16b55dce7733.mp4

Fixed:

https://user-images.githubusercontent.com/43559/197356264-9607883f-106f-4ffe-910f-74c9a0ab8321.mp4

Animation:

Test Build imguizmo test app.

• Click drag track start and end.
• Click expand, add, duplicate, and remove buttons.
• Click drag off remove button.
• Click drag track start over remove button.

[CedricGuillemet]

Thank you!