Prevent flashing redraw of the devices combobox (#696149)

The device combobox was getting drawn blank, then getting drawn again
with the selected device.  This was happening because at the start of
Win_GParted::refresh_combo_devices() the GTK model behind the combobox,
liststore_devices, was cleared, changing the active item, causing the
combobox to get redrawn empty.  After the GTK model had been repopulated
the active item was reset causing the comboxbox to get redrawn again,
now showing the selected device.  Call flow:

    Win_GParted::refresh_combo_devices()
        liststore_devices->clear()
            //Gtk::Combobox emits signal_change.  Registered callbacks
            //called.
                Win_GParted::combo_devices_changed()
                    Win_GParted::Refresh_Visual()
                        ...
        ...
        combo_devices.set_active(current_device);
            //Gtk::Combobox emits signal_change.  Registered callbacks
            //called.
                Win_GParted::combo_devices_changed()
                    Win_GParted::Refresh_Visual()
                        ...

This has always been the case, since the device combobox was first added
to GParted before version 0.1 by commit:

    3a4b43e0adb3974dea656fcbe90d5e191e0e9784
    replaced deprecated OptionMenu with ComboBox ...

Fix by temporarily blocking the devices comboxbox from emitting
signal_changed while the GTK model behind the combobox is recreated.

However, since automatic selection of the largest free space was added
[1] in GParted 0.15.0, a more noticeable flashing redraw issue was
caused in which the partition graphic and partition list were both drawn
blank then redrawn fully populated.  Some distributions were not
affected by this at all, some only experienced a single flash and others
suffered from two or more flashing redraws.  Some affected
distributions: CentOS 5.10, 6.5, 7.0, Debian 6, Fedora 14, 19, 20,
Ubuntu 13.10, Xubuntu 14.04 LTS.  Did not occur on Kubuntu 12.04 LTS.

[1] 5b53c12f6ee12312a9bacb44b8b05b12a540d3d6
    Select largest unallocated partition by default (#667365)

Bug #696149 - Double refresh of display introduced with default
              unallocated space
Reduce flashing redraw from automatic partition selection (#696149)

Automatic selection of the largest unallocated partition caused flashing
redraw of the partition graphic and partition list.  Both the partition
graphic and partition list were being drawn blank then redrawn fully
populated.  This only happened on some distributions including:
CentOS 5.10, 6.5, 7.0, Debian 6, Fedora 14, 20, Xubuntu 14.04 LTS.  Did
not happen on: Kubuntu 12.04 LTS.

This is a workaround, not a complete fix.  It moves automatic selection
of the largest unallocated partition to after processing of the GTK
Event loop in Refresh_Visual() which redraws the partition graphic and
partition list.  These visuals are now drawn only once, fully populated,
however this draws them without the selected partition.  The partition
selection is then drawn afterwards which causes the selection to flash
instead.  This is significant improvement to the whole partition graphic
and partition list flashing when redrawn.

Bug #696149 - Double refresh of display introduced with default
              unallocated space