Defer firing on_select event in GTK OptionContainer

[freakboy3742]

Fixes #2703.

Under GTK, the current_page() API doesn't return the current value while still in the on_select handler; as a result, a user-space event handler trying to retrieve the current tab will get the wrong result.

This change defers the execution of the on_select handler so that current_page() is guaranteed to return the correct result.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[mhsmith]

I can't test this until I've finished updating my Linux machine (toga-gtk requires pygobject >= 3.46.0, which requires system package glib-2.0 >= 2.64.0).