icon-browser: Add scalable icons to icon detail modal window

icon-browser: Add scalable icons to icon detail modal window

When making mockups for GNOME apps in Inkscape, looking for symbolic
icons is a common task. Searching for icons in the file system is clumsy,
and icon-browser provides a much better interface for finding them.
However, currently there is no way to insert the symbolic icons as SVG
directly from icon-browser, so right now it is only useful for finding
the name.

This patch adds a sixth column to the modal window that appears when
clicking a symbolic icon. The icon in this column is labeled "scalable",
and dragging it onto another window results in the vector icon URI being
inserted.

This enables a much simpler workflow when designing with symbolic icons.

https://bugzilla.gnome.org/show_bug.cgi?id=778930