Listener component Editor improvements

[Edvinas01]

• Show a Create button next to the Scriptable Event field in listener components. Clicking this button should show create the appropriate event asset and link it, or should show a dropdown if multiple types are applicable.
• Show a Find (use an icon perhaps?) button next to the Scriptable Event field in listener components. Clicking this button should give a popup which would show all event assets. Selecting an asset should link it to the component. The Find functionality must also provide a search bar. This is a workaround for a bug where you cannot search for generic assets via the object picker (see https://forum.unity.com/threads/generic-scriptable-object-fields.790763/).

[Edvinas01]

Perhaps this could be used:
https://docs.unity3d.com/2020.1/Documentation/ScriptReference/Experimental.GraphView.SearchWindow.html

Props to:
https://www.youtube.com/watch?v=0HHeIUGsuW8

[Edvinas01]

Doing some prototyping in https://github.com/chark/scriptable-events/tree/17-listener-component-editor-improvements

[tvogt]

feel free to steal ideas from my similar project: https://gitlab.com/lemuria.org/observables