[electron][dialog] As a Theia consumer, I want to see native dialogs only when running in electron env

[kittaakos]

Currently, we inject and open Theia dialogs all over the application ignoring the fact that the app might be running in electron env. As a Theia consumer, I want to see native dialogs only when my app is based on electron. Mixing and using native dialogs with the browsers ones is alien. Since we have to remove the remote namespace from the browser code, it would be great to have one generalized dialog service API in Theia that does all the magic for both the electron and browser.

We already have a common API for file-based dialogs:

theia/packages/filesystem/src/browser/file-dialog/file-dialog-service.ts

Lines 25 to 34 in fd23988

	export const FileDialogService = Symbol('FileDialogService');
	export interface FileDialogService {
	showOpenDialog(props: OpenFileDialogProps & { canSelectMany: true }, folder?: FileStat): Promise<MaybeArray<URI> | undefined>;
	showOpenDialog(props: OpenFileDialogProps, folder?: FileStat): Promise<URI | undefined>;
	showOpenDialog(props: OpenFileDialogProps, folder?: FileStat): Promise<MaybeArray<URI> | undefined>;
	showSaveDialog(props: SaveFileDialogProps, folder?: FileStat): Promise<URI | undefined>
	}

electron's dialog API seems to be a good starting point to create a standard dialog API for Theia.

Feature Description: