preferences: use custom context-menu for browser target

[vince-fugnitto]

Feature Description:

Following #7105, the context-menu from the preferences-widget incorrectly uses the native menu when in the browser. This is correct for electron, but in the browser we should use the custom context-menu like the rest of the application:

Browser:

[colin-grant-work]

@vince-fugnitto , could you give a little more information here? Late in the process, we switched from using Phosphor's Menu directly to using Theia's ContextMenuRenderer - is this related to that change?

[vince-fugnitto]

@vince-fugnitto , could you give a little more information here? Late in the process, we switched from using Phosphor's Menu directly to using Theia's ContextMenuRenderer - is this related to that change?

@colin-grant-work I'm sorry, I made the mistake of thinking the dropdown was a context-menu, the fact that dropdowns are native in the browser is an issue across the framework: #4673