Enhance readability of the DialogFeatures constructor

The DialogFeatures::DialogFeatures() constructor used four levels of
nested code blocks and reused the same variable names at multiple
levels.  The variable hbox was used at three of the four levels.  This
was done by commit:

    91b971691d912859cbeb3b916c27bb90c1fcceec
    Bug #634090: Change some attributes to local variables

In my opinion this made the code hard to understand.  Give every local
variable a unique and meaningful name and remove the nested code blocks.
Always show the File System Support dialog legend (#342682)

Make the legend always shown, ready for when the dialog is resizable.

Change the widget containing the legend from an expander to a frame
widget.  Set the frame to be borderless using a bold label as
recommended in the GNOME Human Interface Guidelines 2.2.1 / Controls /
Frames and Separators.

    https://developer.gnome.org/hig-book/2.32/controls-frames.html.en

Bug #342682 - too much information in 'features' dialog
Indent File System Support legend narrative (#342682)

Use padding to indent the legend narrative to better show hierarchy and
association with the "Legend" category as recommended in the:

    GNOME HIG 2.2.3 / Visual Design / Window Layout
    https://developer.gnome.org/hig-book/stable/design-window.html.en

Padding the legend narrative widget also deliberately increases the
spacing to the legend icons too.

Bug #342682 - too much information in 'features' dialog
Make the File System Support dialog resizable (#342682)

Make the dialog resizable, add a vertical scrollbar and set the minimum
(and therefore initial) height to 500 pixels.  This is so that the
dialog entirely fits on an 800x600 screen, thus allowing the rescan
button to be pressed.

100 pixel difference is to account for the size of the top and bottom
GNOME 2 panels and two sets of title bars.  Two sets of title bars
because the window manager tries to place the top of dialog title bars
in line with the bottom of the main window title bar.

Bug #342682 - too much information in 'features' dialog
Restore compilation order of source files

Restore the order of the source files so that they are once again
compiled in order A-Z, a-z.  Order is obtained with:

    fgrep .cc src/Makefile.am | LANG=C sort
    fgrep .h include/Makefile.am | LANG=C sort