Migrate all SelectableText to SelectionArea.

[olegzaim]

Replaced all SelectableText to SelectionArea.
Fixes #698

[guidezpl]

This is not quite what is required. Only ~5 SelectionAreas should be necessary, one per MaterialApp, and one for the setting page, each placed near the top of the widget tree.

Each SelectableText should be replaced by Text.

[guidezpl]

To fix

• Rally text selection is invisible
• Home page doesn't have text selection
• Demos don't have text selection

[olegzaim]

• Rally text selection is invisible.

Hello, there is something wrong with the cursor for selection area because when I select all area it isn't invisible, but when I just tap and press it is stay invisible:

• Single select with long press
  
• Select all
  

• Demos don't have text selection

Is it enough or it's need to be selectable also inside demos?

[guidezpl]

I made some small fixes for consistency. The only thing missing I think is adding selection for the demos (material, cupertino, etc.)

[olegzaim]

I made some small fixes for consistency. The only thing missing I think is adding selection for the demos (material, cupertino, etc.)

Okay, great thank you :)

[guidezpl]

Thanks for contributing!

[guidezpl]

@chunhtai This change triggers errors that cause benchmark and golden tests to fail, would appreciate you taking a look if possible.

[guidezpl]

@olegzaim Could you take a look at the merge conflicts so that we can land this? Thanks!

[guidezpl]

Realized you might only have access to @OlegZaimNLT, if so, please create a new PR cloning this branch