Define icon to use for "button" config item in Designer

Currently, users don't understand the difference between actions and operations in Designer
=> They end up creating an action, then a custom element, when they just want a simple button
=> JSF UI users end up creating actions because they are used to the old "user actions" naming.

We want to rename these options to something more meaningful to help with this problem and provide more distinct icons.

TODO

• Find meaningful icon to use for "button"
• Attach to NXS-4873