Redraw chart when changing to the chart tab

Fixes #59
Force update of the focused widget on the Edit tab if it’s a GtkSpinButton

This is required because when the user changes tabs with an accel after
manually changing a spin button’s value, but without pressing Enter
("activate") or leaving the spin button, the spin button will reflect the
old value instead.
Merge pull request #60 from gergelypolonkai/bug-59

Chart wasn’t updating after manually changing a spin button’s value