Replace widget alignment ALIGN_TOP with ALIGN_CENTER (#652044)

Gtkmm release 2.24 in February 2011 deprecated the use of these
alignment enumerators superseding them with:

    Deprecated enumerator    Use instead
    Gtk::ALIGN_LEFT          Gtk::ALIGN_START
    Gtk::ALIGN_RIGHT         Gtk::ALIGN_END
    Gtk::ALIGN_TOP           Gtk::ALIGN_START
    Gtk::ALIGN_BOTTOM        Gtk::ALIGN_END

    gtkmm: gtkmm Enums and Flags [2.24]
    https://developer.gnome.org/gtkmm/2.24/group__gtkmmEnums.html

The deprecated alignment enumerators were only used in the creation of
Gtk::Label widgets by method Utils::mk_label().  In all but three cases
the alignment was left, center.  So make all labels with alignment left,
center and then stop using the deprecated enumerators.

Step 1 of 3:

ALIGN_TOP was only used for these 2 labels:

    "Path:" text in View > Device Information;
    "Path:" text in Partition > Information.

Use of ALIGN_TOP makes no difference to these labels so just change to
ALIGN_CENTER.

Bug #652044 - uses deprecated APIs
Replace widget alignment ALIGN_RIGHT with ALIGN_LEFT (#652044)

Step 2 of 3:

ALIGN_RIGHT was only used for the size figures in the GParted > Devices
device selector with radio buttons.  Change to ALIGN_LEFT and adjust the
widget packing to maintain appearance.

Bug #652044 - uses deprecated APIs
Stop using deprecated widget alignment enumerators (#652044)

Step 3 of 3:

Now that all label widgets are created with ALIGN_LEFT and ALIGN_CENTER
alignment remove the x_align and y_align parameters from mk_label() and
always use this alignment.  Also specify this alignment via floats
rather than enumerators, one of which was deprecated.

Bug #652044 - uses deprecated APIs