Make it easier to open file dialog

[yohanboniface]

Failed attempt to make it easier to open the file dialog for import:

• tried to add a keyboard shortcut, but it does not work in Firefox (it does in Chrom*, and while it seems it should)
• tried to focus the field and use autofocus, so once the import panel is open one can press Enter to open the dialog, but it does not work either

As I started working on this, I thought this was the opportunity to move the importer panel to a separate class, as we did for the browser panel.