Feature: Add selection RichCommands

[cinqmilleans]

Resolved / Related Issues
*Adds commands SelectAll and ClearSelection and InvertSelection and MultiSelect. The menu is disabled if the command has no effect.

• Adds properties HasSelection and SelectedItem to IContentPageContext for to make it easier. SelectedItem is null when the selection does not contain one and only one element.
• The Ctrl+A hotkey is handled in the file lists and the appropriate command is reissued, to be able to reassign this hotkey.
• Rich commands use a new HotKeyText property, which is null when the hotkey is empty. Otherwise, the menus display None instead of displaying nothing.

Validation
How did you test these changes?

• Built and ran the app
• Tested the changes for accessibility

[yaira2]

Thank you