Ignore backspace events in modal windows (#13180)

Change-Id: Idf3ec4bc80bd015c546000e99cb6ff16e79ca5a8
Merge commit 'd5ce54586a445550b3df5168fe83accb3945901e' into master

d5ce545 Ignore backspace events in modal windows (#13180)

Conflicts:
	client/src/com/vaadin/client/ui/VWindow.java

Change-Id: I22c38a281fac7ed7cfa40f292e6624c5347348fd