New hotkeys and removal of HTML insertion menu

1:1 split       = shift-cmd-0
Hide preview    = shift-cmd-h
Restore preview = shift-opt-cmd-h
Unordered list  = shift-cmd-u
Blockquote      = shift-cmd-b
Link            = shift-cmd-k
Image           = shift-cmd-i

Fix #47, fix #122, fix #124.
Merge hide/restore preview pane items
Merge branch 'new-hotkeys'