Allow specify custom theme version to solve browser cache issues

When modifying custom themes between app releases, browser cache prevent loading the latest version which could mess a lot the vaadin app.
Same thing for widgetset generated JS if the app embed modified custom widgets.
To solve this we should be able to programmatically setup application version and/or theme version so that vaadinBootstrap.js tells the browser to download the latest versions.

For example for theme:
You could replace line 233 in vaadinBootstrap.js:

loadTheme(themeUri, versionInfo && versionInfo['vaadinVersion']);

with

if (versionInfo && versionInfo['themeVersion'])
    loadTheme(themeUri, versionInfo['themeVersion']);
else
    loadTheme(themeUri, versionInfo && versionInfo['vaadinVersion']);

And allow to programmatically set themeVersion somewhere (SystemVersionsProvider like SystemMessagesProvider ?)

For widgetset we could use applicationVersion within versionInfo object.

Start with vaadinBootstrap.js around the loadTheme call and trace how the versionInfo object is produced. Then inspect SystemVersionsProvider and SystemMessagesProvider for the proposed configuration pattern; done means custom theme and application/widgetset versions can be supplied and used for cache-busting.