fix(multiselect): do not process value changes when user input is present
fix(multiselect): do not process value changes when user input is present

Prevents valueChanged event from erasing user input and preventing proper
filter behavior.

All examples verified working in component catalog.

Fixes #515, #516

aurelia-ui-toolkits/aurelia-kendoui-bridge,aurelia-ui-toolkits/aurelia-kendoui-bridge,aurelia-ui-toolkits/aurelia-kendoui-plugin,aurelia-ui-toolkits/aurelia-kendoui-plugin