Add MinGap

[migalv]

Hello, great package!

I was thinking it would be nice to have a MinGap that works as the MaxGap but the opposite. It stretches in the main axis when there is space but can't be smaller than a given size.

This is helpful when you want to separate two Widgets in the main axis but if they don't have enough space they stay at a minimum distance. For example you want to have 2 buttons separated in a row but when the Row shrinks you want them to have a separation of at least 8px, because less than that the widgets would be too close together.