Add context menus in DevTools front-end

[auchenberg]

Add support for context menus that maps DevTools context menus to Electron.

[auchenberg]

Research notes: Take inspiration from https://github.com/microsoft/vscode/blob/94c9ea46838a9a619aeafb7e8afd1170c967bb55/src/vs/base/parts/contextmenu/electron-main/contextmenu.ts