create develop branch

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branch : develop
PencilSetting: delete all shortcuts settings before restore default shortcuts.

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ShortcutsPage: 1. empty lineEdit when shortcuts conflict. 2. fix typo.

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missing file of last commit :/

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ToolSet: ready to remove all changeXXXButton(), merge into methods xxxOn()

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branch : develop
MainWindow2: 1. reconnect Tool's menu action to right slots.
2. rename Editor::clear_clicked() to Editor::clearCurrentFrame()
3. rename Toolset::clearClick() to ToolSet::clearButtonClicked()

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BaseTool: move enum ToolType to pencildef.h

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ToolSet: 1. fix Pen width at 2px. 2. add slot setCurrentTool(ToolType)

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Editor: remove all xxx_clicked() method, replace with Editor::setTool(ToolType)
2. add slot resetView()

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minor

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minor

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ToolSet: remove all unused changeXXXButon() methods after refactoring.

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Merge with develop.
1. Bug fixed: tool shortcuts doesn't work
2. Simplified the tool change related code.
ToolSet: remove unused method

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branch : develop
ToolSet: simplified structure, remove drawPalette.

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ToolSet: remove magnifyButton

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ToolSet: rename varibales.

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ToolSet: remove unused signals, rename m_pEditor => m_editor

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ToolSet: simplified newToolButton()

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MainWindow2: sould not show Preference at beginning.

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branch : develop