Make collapsed panel have flexGrow: 0; call onSaveSizes when collapsing a pane; add units to flex style for resizers

[godsamit]

1. I noticed that collapsed panes will change sizes on container resize... Which happens quite often if you use multiple nested <SplitPane />. As suggested in the issue, there might be something wrong with the width calculation, since an inspection shows the flex-basis of children don't add up to the total container size... But for my use case, adding flexGrow: 0 to collapsed panes solved my problem.
2. onSaveSizes is not called when the user collapse a pane.
3. Added units to the flex property. As discovered here: collapsedSize and buttonPositionOffset adjustments #60

[ysjn]

@b-zurg Hi, do you have time to look into this?
I use react-collapse-pane for my project and having same problem described in #60 #81.

This PR seems to fix them perfectly.
It would be appreciated to merge this fix.

[spencer741]

bump. Would love the merge when you get the chance! @godsamit @b-zurg

[middiu]

any update on this PR, would be really appreciated to have this released