New widget: adaptive modal window

We need a widget which allows us to use modal windows on the desktop, but have them be full screen/full window at small sizes.

In HIG terms, the two patterns this applies to are Action Dialogs, and Presentation Dialogs (https://developer.gnome.org/hig/stable/dialogs.html.en). Message Dialogs are a different matter.

Behavior

• When the window width is smaller or equal to a certain value (likely somewhere around 500px), the modal window should take up the entire area of the window
• This should happen dynamically while resizing
• In presentation dialogs, the "close" button in the header bar should be replaced with a back button on the left side
• The dialogs should come in from the right side with an animation