TODO: AgsSeekable

The AgsSeekable interface needs to be implemented.

Its signal ::seek should be called on the following actions of AgsNavigation:

• prev
• rewind
• forward
• next
• loop

Further notes for AgsEditor and related. The scrolling is really anoying.

No need for this interface wheter notation or pattern both depend on ags-count-beats recall.

And itselves uses AgsTactable interface which shall suit.