Make text selectable for Web

[mono0926]

[mono0926]

It can be supported by using SelectionArea class - material library - Dart API.

• define AdaptiveSelectionMode enum
  • none
  • webOnly(default)
  • all
• Add AdaptiveSelectionMode selectionMode property
  • each function
  • AdaptiveDialog configuration class

Without it, it can be done by wrapping with SelectionArea on builder parameter.

[cbenhagen]

I think this would also be useful on desktop or even mobile platforms.