webadmin: Render widgets more responsive in a generic way

Several widgets have implemented "increased responsiveness" - firing
value change events whenever a key is pressed - independently, while
using similar logic. This patch aims to implement it higher up in the
widget hierarchy, so that this responsiveness could be "turned on" by
a simple method invocation.

Change-Id: Iedad481e878e1a9122d6efec0339bf3ffa4d36ca
Signed-off-by: Lior Vernia <lvernia@redhat.com>