Move chart loading to lazy loading

This is required due to chart preview making, which can take a while.
Make chart/image/icon size parameters for the XSLT; the chart depends on them

Almost all sizes depend on $image_size or $icon_size now
Add an image_size parameter to ag_chart_create_svg()

This is required so we can generate a fixed size image, e.g. for the
chart preview.
Refactor ag_chart_new_full() and ag_chart_add_planets()

…so we can create more generic versions of ag_chart_new_whatever()
Add constructor ag_chart_new_preview()

It creates a new AgChart object with Sun as the only added planet.
Add a built-in, hidden display theme for the preview images

It contains only the Sun, with no aspects nor antiscia
Add a populated parameter to ag_db_chart_save_new()

If the caller intends to fill the whole struct, this should be set to TRUE
Refactor ag_chart_new_from_db() to use ag_house_system_nick_to_id()
Add a preview parameter to ag_chart_new_from_db_save()
Add an icon_size parameter to ag_chart_create_svg()
Outsource chart pixbuf creation to ag_chart_get_pixbuf()
Generate preview images of charts for the list view
Merge pull request #93 from gergelypolonkai/chart-preview

Generate preview images for the charts in the list view