Add Search box is toggled with search button

[lulol]

Search box is hidden by default and is shown pressing the magnifying glass icon of the search button. When the search box is empty it can be hidden again pressing the search button.

Now should be less confusing and certainly is less ugly.

[clefebvre]

Nice!

• Can you use a toggle button instead of a simple button?
• If you search for channels, you can't hide the search bar... the button should let you hide the bar even if you want to stay in the result screen, as it is it looks broken because it doesn't do anything.

[lulol]

Ok. I think that can do this:

• The toggle button should act just to hide/unhide the search box and don't trigger a search.
• Search box could be hidden while containing the search text, which still will be shown in the headerbar.
• To trigger a search must press ENTER as the button only will toggle the search box visibility.
• Also, CTRL-F should act as a toggle for the search box?

After you comments yesterday was looking at using Gtk.SearchBar that provides such a sliding search box. The Shortcuts window uses it but seems that it's implemented directly by the toolkit so couldn't peek directly at how was done. Seems that it needs Gtk.Grid and I think that would need extensive modifications to the rest of the layout and I'm not knowledgeable enough about GTK+.

[lulol]

Have rewritten the search code using Gtk.FlowBox.set_filter_func() that seems faster and the proper way to do it instead of destroying and recreating childs unnecessarily (and triggering more network request failures for still missing logos).

Will update soon after a bit more testing with some other of the huge provider lists to verify that all works as expected.

Should I close this PR and start a new one, or is better to just squash all the commits?