Load workflow selector after UI is rendered

**Is your feature request related to a problem? Please describe.**
One of the reasons why App loading is so slow is that some widgets interact with the DB during initialization.
One big offender in this area is the WorkChainSelector widget, which takes quite some time to load (and it gets worse with the number of workflows).

**Describe the solution you'd like**
It makes sense to first load the app and then load the workflow selector in the background.
I have already implemented this in my Atmospec app and it works great (and the code change is small). Please see this PR

https://github.com/ispg-group/aiidalab-ispg/pull/216

**Describe alternatives you've considered**
An alternative would be to improve the widget so that it does not load _all_ the workflow at once. But then we would need to end some kind of filtering based on a date (and possibly other parameters). That's definitely something to look at in the future.

CC @unkcpz this is what I mentioned on the call today. I thought I have already created this issue but apparently not. :-)