Better Touchscreen Support

Description

This PR adds support for gestures. Allows touchscreen input to move the cursor, scroll, select, and open the context menu.

Gestures

• When scrolled to the end of output (or mouse-tracking enabled),
  • Two-finger right-swipe: Tab
  • Two-finger left-swipe: Escape
  • Single-finger pan: Arrow keys
• Not scrolled to the end,
  • Vertical, single-finger pan: Scroll up and down
  • Horizontal, single-finger pan: Select
• Any scroll,
  • Two-finger pan: Vertical scroll
  • Long tap: Right click
  • Short, 1-finger tap: Left click
  • Short, 3-finger tap: Middle click
  • Short tap, drag: Left-button down, move, end mouse event.

To-do

• Tests
• Kinetic scrolling
• Drag and drop on long tap -> drag
• Dolphin-like right-click indicator

Screencasts

Notes

• Initially, I tried to use QGestureRecognizer. Unfortunately, it only recognized two-finger gestures.
• It might be helpful to compare this PR to this PR that improved touch support in Dolphin.
• The code is currently somewhat messy...

BUG: 437553