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
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.
Country selection is now saved, so city list can be filtered
Unify AgChart API, so ag_chart_get_*() that return strings are const
Set header subtitle to the chart name upon updating

Fixes #61
Merge pull request #63 from gergelypolonkai/bug-61

Show chart name in header bar upon loading
Add a "retrograde" attribute to <body> tags in the generated XML
Update chart-default.xsl to show retrograde mark
Fix accel for chart edit

For some reason it was the same as the gear menu
Updated libgd dependency
Fix accel for chart edit

For some reason it was the same as the gear menu
Updated libgd dependency
Fix retrograde mark location
Merge pull request #64 from gergelypolonkai/retrograde-mark

Mark retrograde planets on the chart
Merge branch 'master' into city-chooser