Set <Control>G as the accelerator for the 'Go to Star' button

Make it possible to activate the 'Go to Star' button with either the
<Control>G accelerator or the <Alt>G mnemonic key. In this manner, the
user can quickly show and hide the Finding Chart window (<Control>F)
and view the details of the selected star (<Control>G).
Add unit test for FITSImage.update_keyword()
LEMONdB.get_photometry(): cast star ID to Python's built-in int

Avoid the "sqlite3.InterfaceError: Error binding parameter - probably
unsupported type" exception raised by LEMONdB.get_photometry() if the
star ID is a NumPy integer and not the expected Python's built-in int.
Thanks to Javier Blasco for reporting this.
FITSImageTest.test_update_keyword(): re-read FITS file from disk
Add unit test for FITSImage.delete_keyword()
Rename column 'Mag' to 'Δ Mag' in the list of points of each curve
Remove extra spaces around the assignment operator
Add a 'View in Finding Chart' button to the star details window

Use a stock button, STOCK_JUMP_TO (an arrow pointing downwards) but
with a different label, 'View in Finding Chart', by getting the label
child and setting the text directly. The button is not yet functional.
LEMONJuicerGUI.view_finding_chart(): add 'set_visibility' keyword arg
Rename LEMONJuicerGUI.view_finding_chart() to handle_finding_chart()
Rename FindingChartDialog.mark_star() to mark_closest_star()
Mark a star when the user clicks on 'View in Finding Chart'

Add a new method, StarDetailsGUI.handle_view_star_in_chart(), and
connect to it the 'clicked' signal of the 'View in Finding Chart'
button. This method shows the gtk.Dialog, if not already visible, and
in turn calls the FindingChartDialog.mark_star() method, overlaying a
green marker (#24ff29) on the star.