Scrollable-reorderable editor tabs bar

[tomlin7]

Feature

The current editor tabs bar is not scrollable, this affects visibility of tabs when many are opened, and makes it unusable.

Notes

• Tab bar frame should be replaced with a canvas: biscuit/core/layout/base/content/editors/tabs

• A horizontal thin scrollbar can be added below/above the tab bar (opinions? it may not be possible to make a thin scrollbar)

• Horizontal scrolling with mouse scrollwheel

• Now for reordering part, this might be somewhat hard to achieve. Can use the native tkinter.dnd handler here.

• Limit the draggability to horizontal only.

• Highlight tabs when another tab is dragged over it, so to indicate that tab's position will be taken over by new one