Add 'Export' button to dump the light curve points to a text file

Include below the list of points of the light curve a gtk.Toolbar with
a single button, 'Export'. When clicked, it displays a message dialog
that allows the user to select which columns of the GtkTreeView (i.e.,
which attributes of the light curve) will be exported to a plain-text
file. The date of observation can be saved in two different formats:
seconds since the Unix epoch or the 24-character representation
returned by the time.asctime function. The path of the file to which
the data is dumped is selected using a gtk.FileChooserDialog, brought
up when 'Save' is clicked.
Remove unused class variables in AmplitudesSearchMessageWindow
Make the number of decimal places in exported curves configurable

Use a spin button to allow the user to select how many decimal places
are used when the light curve is exported to a plain-text file. This
spin button has a minimum value of zero (i.e., floats are rounded to
the nearest integer), maximum of eighteen and defaults to eight.
Save and read curve export options to and from the configuration file

Use an event handler for each button in the dialog window (60fe26b4de)
to update the in-memory copy of the configuration file every time the
user changes which attributes of the light curve will be exported to a
plain-text file or the number of decimal places that will be used. In
this manner, the options for how light curves are dumped to text files
persist across different executions.
Assign sequential numbers to amplitudes–wavelength searches

Although, since commit a28012c0e8ca182, AmplitudesSearchPage instances
include a description on the left, they still display the same text in
the tab of the gtk.Notebook: 'Amplitudes ↑' or '↓', depending on how
amplitudes are directly or inversely correlated with the wavelength,
respectively. Use the Roman notation to number the searches made in a
LEMONdB sequentially: the first one is labeled with 'Amplitudes I',
the second, with 'Amplitudes II', and so on.
Always sort columns by wavelength in AmplitudesSearchPage

Matilde Fernández suggests that columns in the AmplitudesSearchPage
instances must always be sorted by the wavelength, so that the user
can easily appreciate how amplitudes increase or decrease with the
wavelength. This approach has the advantages that the photometric
filters are listed in the same order regardless of the type of search,
and that it matches the terms used in the description on the left.
Remove repeated word
Set mnemonic accelerator key for the 'Find' menu to 'i'
Add AmplitudesSearchPage.toxml method

The method returns the standalone XML representation of the object,
ready to be written to disk. This method will be used to allow the
user to save a search results page to a file so that it can be loaded
back later to resume the work, as suggested by Matilde Fernández.
Factor out duplicate code into the xmlparse.setup_header function
Remove the parent gtk.GtkBuilder from AmplitudesSearchPage.__init__

Do not require the instantiation method of AmplitudesSearchPage to
receive as its first parameter the gtk.GtkBuilder of the parent GTK
widget. We do not need to share objects between the two widgets, so
the code is cleaner (even if only a tiny bit) if we create our own
instance of gtk.GtkBuilder in the class.
Add AmplitudesSearchPage.xml_dump (writes XML representation to disk)
Fix misspelled word
Add 'Save As' button below the description in the AmplitudesSearchPage

Allow the user to export the search results to an XML file, format
which is not only machine-readable (and with which serializing and
de-serializing the AmplitudesSearchPage object is made easier) but
also human-readable. A gtk.FileChooserDialog shows up when the button
is clicked and lets the user browse for the location where the file is
saved. The dialog suggests a name based on the label assigned to the
AmplitudesSearchPage in the LEMONJuicerGUI's gtk.Notebook: lower case,
except for the Roman numerals, and replacing whitespaces with dashes.

Note that, now, the AmplitudesSearchPage.get_label method stores the
last returned title in a class attribute, 'label', so that is can be
used later by the export_to_file method to suggest the filename. This
attribute is not initialized at __init__, so accessing it before the
method is called for the first time will raise the AttributeError
exception. This attribute is needed because the title of the search is
not inherent to the AmplitudesSearchPage, but depends on the number of
searches previously made, track of which is kept by the 'parent'
class, LEMONJuicerGUI.
Show stock icons in buttons ('gtk-button-images' property)