Add copy/paste and duplicate for animation clips

[ambrosiogabe]

We should have copy paste working. If there's an active object selected and an active timeline clip selected:

• If the user is hovering over the timeline, copy/paste the selected clip

If the mouse is hovered over the timeline, paste the clip where the mouse is hovered. Otherwise, paste it directly over the parent.

Copy/paste menus should also be available. Right-clicking the clip in the timeline should popup the menu offering to copy/paste.

When pasting from a menu:

• The clip should be placed in a non-empty area. If it can't fit in the current track, then a new track should be created and it should be placed there.
• When right-clicking the timeline to paste, the pasted clip should go where the cursor is (on a new track if necessary).