Table.on_activate should fire when pressing Enter with the table focused

[mhsmith]

It currently only fires when double-clicking, or long-pressing on mobile.

See discussion #2705.

[cgkoutzigiannis]

Based on the GTK docs on TreeView this is the default behavior on Linux. So we should only look to implement this on Windows and macOS.

[freakboy3742]

@cgkoutzigiannis If it is the default behaviour, it may not require additional handling in the GTK backend, but we still require a testbed test to validate that the behavior is implemented as a default (if only to protect against a future GTK update where they remove that default).