gesture for click + hold back button

[nktnet1]

Feature request from email:

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With 1 (gesture for click & hold back button), it may be possible to implement, just tricky since the method of detection varies by devices and Android versions. May want to reference Firefox's implementation.

Potential issues to note:

• https://www.reddit.com/r/firefox/comments/1hyujo4/hold_back_button_for_tab_history_has_stopped/
• https://bugzilla.mozilla.org/show_bug.cgi?id=1932300

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With 2, likely a limitation with Android pinned mode.

[nktnet1]

Resolved by #58.