Unify the iconview and listview modes of the Media Library into a single responsive grid view

Submitted by Jeff F.T. @jeff

Description

Pitivi's media library has two view modes to accommodate different types of screens:

• Listview is better-suited to smaller resolutions (ex: 1280x*, 1024x*).
  We show smaller thumbnails.

• Iconview leverages high-res wide screens (ex: 1920x* and bigger) better,
  as its items will wrap horizontally.

Iconview has rather annoying bugs and lackluster performance.
Having both iconview and listview/treeview complexifies the code and the user experience.

When possible, we should switch to a "responsive" grid widget in GTK (bug # 704722).