Jump to bookmark table shortcut

[rzvxa]

Description of Changes

Closes #1304 and #1307

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New Version Info

Adds the 'gb' binding to jump to the bookmark table.

[rzvxa]

@alerque Hi, It is based on #1307. I've made some clean-ups here and there and changed the shortcut to gb instead of b to prevent clashing with the back vim motion. This implementation will jump to the bookmark table if it is already open, and will open it when it is closed.

Let me know whether you want this to be included in 7.1.0 or 7.2.0.