MainWindow2: remove ununsed action 'Extend Frame'

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Prepare to implement custom keyboard shorcuts, remove all hard-coded shortcuts.

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PencilDef: define command constatns for keyboard shortcuts

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MainWindow2: create method loadShortcuts()

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add default shortcuts setting file

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Merge

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README: pencil -> pencil2d, add Morevna's release note
README: minor
Merge with default

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branch : customkb
MainWindow2: remove unused method loadPlugins()

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branch : customkb