Update pull and export tab forms list while forms are being pulled

[ggalmazor]

This issue is related to #463

Software versions

Briefcase v1.10.0

Problem description

When a user selects more than one form to pull, the push and export tabs will update their form tables only when all the forms and their submissions are pulled and the cache is updated.

Since that can take some time, it can be confusing to see that some of the forms have been successfully pulled, but don't find them in the push/export form tables.

Steps to reproduce the problem

Select some forms and pull. Switch to push/export tabs and see how no form shows up until all of them have been pulled.

Expected behavior

We could explore different options:

• Block the push and export tabs until the pulling is done
• Add some kind of "pulling forms, please wait..." message

Other information