[Launchpad] Search filter input regularly steals focus

[fvsch]

Steps to reproduce:

1. Start Launchpad
2. Launch Firefox, and open a few tabs
3. Switch back to Launchpad. Keyboard focus will probably be in the "Filter tabs" input
4. Hit the Tab key to put the focus on any item in the tab list
5. After a few seconds, focus goes back to the "Filter tabs" input

It looks like we are regularly re-rendering the whole content, with the header+input too, and the render function is probably calling .focus() on the input.

The solution would be to re-render only the tab list.