Support for multi-select in widget.List

[sdassow]

Description:

Add support for MultiSelect to widget.List, partially based on #4477 but sticking with the API outlined in #2575 as a starting point.

API changes for widget.List:

• New attribute: MultiSelect bool
• New method:SetSelection(ids []ListItemID)
• New method: SelectAll()

Some design decisions:

• Toggle selected/select multiple items:
  • darwin: Cmd+Space/Tap
  • linux/windows: Super/Ctrl+Space/Tap?
  • mobile: SecondaryTap
• Range selection with Shift+Space/Tap from nearest selected item
• Range increase/decrease with Shift+Up/Down

Partially fixes #2575

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: 60.204% (-0.1%) from 60.32% — sdassow:feature/list-multi-select into fyne-io:develop