Feature: flag to hide empty tabsets but not delete them

[PimpMX]

Hi, we're currently developing a product based on the FlexLayout and we have a specific use case where we either have to be able to add new tabsets (currently there is no action that supports this) or be able to define a layout which contains empty tabsets, which serve the purpose of being placeholders for future insertion of tabs through actions.

This serves the purpose of being able to programatically open tabs (in our placeholder tabsets) left and right of our main tabset and have them appear only when they're not empty. In our case we have a pdf editor in the center tabset of the row node, and we'd like to be able to split documents or open other documents left and right of our main editor tab and also be able to apply application state (stored in the url query string, we store which documents are open and how the editors are aligned in the layout) at startup time or when receiving a message from our MQTT broker

To demonstrate what we're looking for and for further discussion, i've created this fork which introduces our desired functionality

Fork-Changes

• Introduced a new boolean global var "tabSetEnableHideWhenEmpty" (tabSetEnableDeleteWhenEmpty's little brother)
• Introduced a new optional boolean attribute on ITabSetAttribute called "enableHideWhenEmpty" which inherits from "tabSetEnableHideWhenEmpty"
• Added a new layout called "ecmind" which has a row node which contains three tabsets, the left and right one are empty and the middle one is filled. The layout has "tabSetEnableHideWhenEmpty" set to true
• Adjusted the Row.tsx so that tabsets that are empty do not get rendered when they have no children and "enableHideWhenEmpty" is set to true
• When starting the demo and selecting the "ecmind" layout two additional buttons appear next to "Add Active", "Add to left empty Tabset" and "Add to right empty Tabset" clicking these inserts a tab into the empty tabsets, demonstrating that the tabsets appear once they're not empty anymore