[Epiphany] Tabs changes

[Marco Pesenti Gritti <marco gnome org>]

So here it's a list of concrete changes I'd like to make to tab
handlding in the next days:

- Remove the confusing "Open in tabs by default" pref
- Make middle click on link open in new tab by default
- Remove "Open in new tab" menu from history and bookmarks. It's
confusing because you dont know in which window it will be opened. (a
bit like links, just a bit less because you opened the dialog from a
window). Instead make Open in new window use tabs to open all the urls
in a multiple selection. That way we provide the ability to open a
group of urls.
- Add an hidden pref for ever show tabs. Not a big cost, let's keep it
until we have a better solution.

Hope it make sense.

Marco