Release v0.2.0

This release contains the following changes:

• [ADD] Add shortcut buttons to halve/double the pixel size unit
• [ADD] Support flip and rotate operations
• [UPDATE] Brighten unfocused text box background color
• [UPDATE] Merge multiple redraw requests issued during an event handling
• [UPDATE] Change the pick-button into a non-clickable state when it's selected
• [UPDATE] Remove erasing tool dialog as it's redundant with the selection-then-cut feature
• [FIX] Ensure imported image positions align with the pixel size
• [FIX] Consider pixel size when copying