WIP: Improve scroll speed

[sdassow]

Description:

First attempt at improving scroll speed by reducing the amount of calls to Draggable.Dragged.

Big diff because it introduces an interface type for queued events to allow differentiation for optimization purposes.

References:

• Scrolling is laggy with high speed gaming mice #4997
• widget.List is very slow on android #4058
• Android apps are lagging when enter text and scrolling #1062
• Fyne splitters & scrollers lag behind commands when there is too much content on screen #3374

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

Where applicable:

• Public APIs match existing style and have Since: line.
• Any breaking changes have a deprecation path or have been discussed.
• Check for binary size increases when importing new modules.

[coveralls]

coverage: 59.947% (-0.05%) from 59.996%
when pulling a9a21a9 on sdassow:improve-scroll-speed
into 6bfd026 on fyne-io:develop.