chore: Improve 'Add to playlists' dialog

[natilou]

Fixes #344

[dweymouth]

Could we show all the playlists initially instead of an empty list? Note that this behavior would be different from the Quick Search dialog so it would probably need to be handled specially in the Add to playlist dialog implementation

[natilou]

Yes, I can add a list with all the playlists and then filter by the search text

[natilou]

I overrode the Show() method from SearchDialog to populate the results list with all the user's playlists, but I have to call that method in the controller:

m.ClosePopUpOnEscape(pop)
m.haveModal = true
min := sp.MinSize()
height := fyne.Max(min.Height, fyne.Min(min.Height*1.5, m.MainWindow.Canvas().Size().Height*0.7))
sp.SearchDialog.Show() // I added this line to call Show()
pop.Resize(fyne.NewSize(min.Width, height))
pop.Show()
m.MainWindow.Canvas().Focus(sp.GetSearchEntry())

I'm not sure if this implementation is correct or if there is a better way to do it.

[dweymouth]

I'm going to merge this into a feature branch to test and tweak a bit before going to main, hope you don't mind :)