bookmark-properties-grid: Change to a box container and rename widget

Currently the EphyBookmarkPropertiesGrid uses a GtkGrid to layout the
different widgets. But it only uses a single column, which means the
grid is basically useless as a GtkBox would do the job better without
requiring the different packing properties (as long as the childs are in
increasing order).

So this commit changes it to a GtkBox subclass and renames the widget
accordingly, without including "box" in the name as it's not useful.
3 jobs for fix-bookmark-properties in 9 minutes and 37 seconds
Status Job ID Name Coverage
  Test
passed #1346474
check-code-style

00:01:11

passed #1346475
flatpak
flatpak

00:07:36

passed #1346476
flatpak
scanbuild

00:09:35