gtk: Create the widgets in the background before inserting them

Previously we were creating the widgets and inserting them at the same time. While we want to batch the insertions to avoid triggering multiple stylesheets invalidations, we can create all the widgets in the background at once, and then only start batchign the insertions.

This way we reduce significantly the time we block the main thread when creating widgets, as we can yield after each one, while maintaining the batching of widget insertions into the containers.