Combine all types of bookmarks in one place

[narayan]

x2 actually has THREE different types of bookmarks:
1. Single bookmark (load one tab in active pane)
2. Dual bookmark (load one tab in each pane)
3. Folder group (load a set of tabs in active pane)

Since the purpose is the same, they should be accessible from a single ("BOOKMARK") menu.

In particular, there is no need to provide the dual bookmarks in a separate menu. Let them be mixed with the single bookmarks. The two varieties already have their different icons, so we can easily tell them apart.

@Why "Folder group" menu should be combined:
First of all, the name "folder group" is confusing, because it obviously means a set of folders (as in a few folders collected in a scrap pane). Semantically, the name "Tab group" is much better.

But it would be BEST to shift them to the bookmark menu.

Only one streamlined menu will do, with a small change here:
Bookmarks>Add current> Current tab
Bookmarks>Add current> Set of tabs
Bookmarks>Add current> Dual tabs

The rest of the menu options under Bookmark remain undisturbed.