`Tabs` layout should be configurable by call site

[PavelHolec]

Tabs should let the call site define the sizes of tabs or the whole component.

By default:

• Tabs would itself be idealSize = false, horizontally expanding.
• each tab would have idealSize = false too, expanding

A call site could:

Tabs {
   tab1
   tab2
}
.idealSize()   // each tab would have expanding size, but overall the Tabs component would not be expanding

Tabs {
   tab1
       .idealSize()   // this tab would only have its ideal size fixed
   tab2
}