Merge lp://staging/~elementary-pantheon/switchboard/flowbox into lp://staging/~elementary-pantheon/switchboard/switchboard

It works & looks great for me, besides those issues that are pointed out in the description.

I personally don't like how it splits the distance equally between icons, it leads to a huge gap between them. Furthermore, it seems that even if there is enough space, the Wacom tablet icon in the Hardware flexbox is still saying on a separate line.

It seems if you have two rows of icons in "Personal", it only skips the first row, so it goes to the first icon in the second row. Maybe there's some overlap?

Below is the log of a gdb session where you can see the stacktrace of where that g_sequence_get assert failure is happening:

(gdb) bt
#0 0x00007ffff60063c0 in g_sequence_get (iter=0xc0c4c0)
    at /build/glib2.0-t9oPgV/glib2.0-2.48.0/./glib/gsequence.c:1186
#1 0x0000000000411e48 in switchboard_category_has_child ()
#2 0x000000000040edd4 in switchboard_category_view_filter_plugs ()
#3 0x000000000040e793 in switchboard_category_view_add_plug ()
#4 0x000000000040e10b in __lambda11_ ()
#5 0x000000000040e15c in ___lambda11__gsource_func ()
#6 0x00007ffff5febfda in g_main_context_dispatch (context=0x666cb0)
    at /build/glib2.0-t9oPgV/glib2.0-2.48.0/./glib/gmain.c:3154
#7 0x00007ffff5febfda in g_main_context_dispatch (context=context@entry=0x666cb0)
    at /build/glib2.0-t9oPgV/glib2.0-2.48.0/./glib/gmain.c:3769
#8 0x00007ffff5fec380 in g_main_context_iterate (context=0x666cb0, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at /build/glib2.0-t9oPgV/glib2.0-2.48.0/./glib/gmain.c:3840
#9 0x00007ffff5fec6a2 in g_main_loop_run (loop=0xc96f40)
    at /build/glib2.0-t9oPgV/glib2.0-2.48.0/./glib/gmain.c:4034
#10 0x00007ffff6a9f6f5 in gtk_main () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#11 0x0000000000407640 in _switchboard_switchboard_app_command_line ()
#12 0x0000000000406a62 in switchboard_switchboard_app_real_command_line ()
#13 0x00007ffff1c42e40 in ffi_call_unix64 () at /usr/lib/x86_64-linux-gnu/libffi.so.6
#14 0x00007ffff1c428ab in ffi_call () at /usr/lib/x86_64-linux-gnu/libffi.so.6
#15 0x00007ffff62c3cf5 in g_cclosure_marshal_generic_va (closure=0x70b480, return_value=0x7fffffffd9d0, instance=0xa8e150, args_list=<optimized out>, marshal_data=0x4069e4 <switchboard_switchboard_app_real_command_line>, n_params=1, param_types=0x70b4f0) at /build/glib2.0-t9oPgV/glib2.0-2.48.0/./gobject/gclosure.c:1604
#16 0x00007ffff62c31d4 in _g_closure_invoke_va (closure=closure@entry=0x70b480, return_value=return_value@entry=0x7fffffffd9d0, instance=instance@entry=0xa8e150, args=args@entry=0x7fffffffdaa0, n_params=<optimized out>, param_types=0x70b4f0) at /build/glib2.0-t9oPgV/glib2.0-2.48.0/./gobject/gclosure.c:867
#17 0x00007ffff62dd4b8 in g_signal_emit_valist (instance=0xa8e150, signal_id=<optimized out>, detail=0, var_args=var_args@entry=0x7fffffffdaa0) at /build/glib2.0-t9oPgV/glib2.0-2.48.0/./gobject/gsignal.c:3294
#18 0x00007ffff62de08f in g_signal_emit (instance=instance@entry=0xa8e150, signal_id=<optimized out>, detail=detail@entry=0) at /build/glib2.0-t9oPgV/glib2.0-2.48.0/./gobject/gsignal.c:3441
#19 0x00007ffff65b1a13 in g_application_call_command_line (application=0xa8e150 [SwitchboardSwitchboardApp], arguments=<optimized out>, options=<optimized out>, exit_status=0x7fffffffdcd4)
    at /build/glib2.0-t9oPgV/gl...

Hm so it looks like it's throwing a fit over this line:
"foreach (unowned Gtk.Widget child in flowbox.get_children ())"

Can confirm that this solves the HiDPI icon issues. Glorious, crispy icons. Under fairly normal window sizes, it appears to be working as I'd expect.