Default size with pixel value

[StevenStavrakis]

Describe the feature in detail (code, mocks, or screenshots encouraged)

Sometimes it is more useful to have a pane start at a static size that doesn't respond to resizing of the entire PaneGroup. The ability to pass a string into defaultSize instead of an integer would allow devs to specify what units they want instead of defaulting to percent.

What type of pull request would this be?

New Feature

Provide relevant links or additional information.

No response

[huntabyte]

It won't be added to this project anytime soon, see this issue for more information: #11 (comment)