Make file dialogs work in <webview>, fixes #930
Add BrowserWindow.showDefinitionForSelection()

This API shows the system-provided pop-up dictionary.
Some Mac apps including Chrome have "Look Up in in Dictionary" context
menu item. This API can be used to implement it.
Merge pull request #938 from omo/show-definition-for-selection

Add BrowserWindow.showDefinitionForSelection()
Small style fixes