Performance: Don't launch an initial search on an empty SearchEntry

[nekohayo]

Describe the bug

Currently, the app launches a search immediately the first time you switch to the "Browse" tab, even if the user hasn't typed anything, and this empty search takes a very long time (ex: 30 seconds or so) with the spinner active throughout this time. This should not happen:

Screencast.from.2024-07-25.07-52-19.mp4

System Information:

• Distro: Fedora 40
• Extension Manager version: 0.5.1
• Package format: Flatpak

[oscfdezdz]

For context, this happened because the servers were down for maintenance and apparently it took longer than expected and affected other services, see https://discourse.gnome.org/t/planned-maintenance-ocp-cluster-25th-of-july-2024-8-am-12-pm-utc/22321/8

Regardless, as you mentioned in #672 (comment), we should show a warning when there is a problem with the servers.

To avoid launching an initial search when switching to the Browse tab, maybe we could cache the most popular extensions from time to time. Although it seems that the Extensions website does an initial search every time, just like we do.