TimelineFramesHeader should be always kept in view

[Casqade]

TimelineFramesHeader view is constrained only by its left border. User isn't allowed to drag the view left past range's first value, however dragging right past last value has no limits. There aren't any buttons to reset the view to value range (#1169), so it's very easy to scroll past all frame bars and lose track of where they are on the timeline.

Mouse zooming also suffers from the same issue:

1. Zoom out to min zoom level
2. Move mouse close to right screen border
3. Zoom in
4. Frame bars are now lost

Another one:

1. Zoom to fit at least all frame bars on the screen
2. Move mouse close to the right border
3. Zoom out. View constraint shifted all bars left, thus changing the zoom pivot
4. Zooming in again results in a different view from step 2, even though the mouse didn't move. Frame bars are probably out of view now

I don't know what's the best way to improve it, here's my thoughts:

1. Allow panning the view left, past the first frame. This way zoom pivot will always be consistent
2. Same as 1, but constrain panning by last frame. So if whole range fits the screen, don't allow to drag it beyond the screen borders. Same goes for panning right
3. TimelineFramesHeader and zones view panning could be synchronized in some way
4. Don't allow TimelineFramesHeader to be zoomed-out/shrinked if its first/last values already occupy full screen width (in that case it always scales to screen width and doesn't allow to scroll past the range)