Adapte the modal dialog the viewport height

This PR rework the CSS to display the scrollbar inside the dialog instead of the use of the window scrollbar.

Plus a smooth transition between the empty and feed form.

As result:

• Title is always visible
• Validation buttons are always visible

The transition is managed with max-width, which is a kind of trick. The down side is

• there is no way to have a smooth transition from feed to empty. For example when you select another scan.
• and it enforce to use a hard coded value (i have used 999).

But this could be improved later.

Here is with a big scan form of mine

Here is with the new sample form

You can see that the window scrollbar is still sometimes displayed, but the main issue with big form is fixed. So i don't really care.