Allow the Splitter to have only sized panes

[vveesseelliinnaa]

Currently the Splitter requires to have at least one pane with no set size, otherwise the component throws an error:

• https://stackblitz.com/edit/react-li5c8p?file=app%2Fmain.jsx

In some scenarios, the client needs to set the size of all panes so the whole Splitter size is increased when a pane is resized instead of changing the size of the other panes.