Slate 0.4.0

• Added support for 90 degree rotation.
• Added logo/application icon.
• Added support for importing and exporting swatches (#82).
• Added flip horizontally + vertically tool buttons to tool bar.
• Added "About Slate" menu item.
• Improved documentation.
• Ensure that we prompt to discard changes before quitting on macOS.
• Increased width of non-native recent files menu so that it's possible to see more of the path.
• Only enable the "Show Grid" menu item for tileset projects (#80).
• Sped up pinch zooming on macOS.
• Fixed gap between panels after switching project types.
• Uppercased the S in executable name.