These are being replaced by GtkFileDialog.
This commit only moves the headers for GtkFileChooserWidget and
GtkFileChooserDialog to deprecated/, and keeps the implementations
in gtk/, since they will eventually be salvaged into a private
GtkFileChooserWindow.
The "Show Time" setting does not take immediate effect (only after
changing folders) because it's set as a single call to
column_view_get_time_visible() on the FileChooserCell creation.
Instead create a bind a show-time property that gets updated
as the setting is changed.
Move the gestures to the individual cells, and
make them trigger the context menu via an action
that takes item position and coordinates.
The semantics are changed slightly: the menu actions
now operate on the clicked item, not on the selection.
Still to do: Fix up keyboard activation.
