V5touch: add V5-like touchscreen keypad to the UI

[wanthalf]

Touchscreen V5 keypad implementation as discussed in https://discourse.zynthian.org/t/diy-upgrading-to-v5-or-the-question-of-new-buttons/9707

Requirements and options:

• python3-cairosvg to support SVG icons (should fall back to PNG if not available)
• environment variable ZYNTHIAN_TOUCH_KEYPAD set to V5 to activate this extension (without this setting, the UI should behave exactly as before)
  -optional environment variable ZYNTHIAN_TOUCH_KEYPAD_SIDE_LEFT to activate the left side layout of the keypad (TRUE by default)
• optional environment variables for setting custom onscreen buttons foreground colours, labels etc.