Disable Close button and menu item when there are no pages open
Set a minimum window size of 1024 x 768 pixels
Prevent dates from overlapping on the x-axis

Matplotlib allows us to natively plot datetime instances. Although for
the most part it does a good job picking tick locations and string
formats, these are sometimes squashed together. In order to avoid
this, we can set the maximum number of x-axis ticks to eight, a value
which seems to work well with our minimum width of 768 pixels.

Note that the maximum number of ticks ('maxticks' keyword argument in
matplotlib.dates.AutoDateLocator.__init__) was not available until
version 1.0.0 (July 2010). Nothing has been implemented in our code
to deal with older versions, so dates may overlap in those cases.
Show the main information of the star

Add a third gtk.TreeView to the page with the details of each star,
using it to display the same information that is shown in the list
of stars of the LEMONdB: image and sky coordinates, instrumental
magnitude in the reference image and the period in the different
photometric filters.

The headers of the TreeView are not visible; we just need a simple
table of two columns, where the first has the name of the attribute
(such as "Right ascension") and the second the corresponding value
("06h 41m 04.41s", for example). Showing the generic headers
"Attribute" and "Value" would not add any useful information.

This first version does not yet take into account the options selected
in the View menu. Therefore, coordinates are shown in both sexagesimal
and decimal formats, and each period in three different units: days,
hh:mm:ss and seconds.
Set page size of spin button's adjustment to zero

This stops the "GtkWarning: GtkSpinButton: setting an adjustment with
non-zero page size is deprecated" from being raised. According to the
GTK documentation, the page size is irrelevant and should be set to
zero if the adjustment is used for a simple scalar value, e.g. in a
GtkSpinButton.
Select the unit of the periods also when star details are shown

Implement the functionality of the View — Coordinates submenu in the
StarDetailsGUI class too, so that three different formats: days
(0.8889 days, for example), hh:mm:ss (21:20:00) and seconds (76800)
can be chosen.

In order to be able to hide some of the rows of a TreeView, we need to
use a TreeModelFilter. This class acts as a wrapper for our TreeModel
(a ListStore or TreeStore), allowing us to decide which rows are
displayed based on the value of a column with boolean values. Thus, we
need to add a third and last column to the store, which controls the
visibility of the period rows and whose values are updated when a
different unit is selected.
Allow to select the format(s) of the star coordinates in StarDetailsGUI

Implement the functionality of the also View — Coordinates submenu,
which lets the user choose whether sexagesimal coordinates, decimal
or both are shown.
Fix incorrect method call (wrong name of keyword argument)
Do not use subprocess.check_call to run photometry.py

Now that the PyRAF process-caching memory is turned off (see commit
b7c650d174), photometry.main() can be directly invoked. We were using
the subprocess module as a workaround for what we erroneously thought
was a bug in PyRAF, running photometry.py separately so that it did
not 'corrupt', as it seemed, our Python namespace.