Add the ability to generate a header for GdkRGBA.

GdkColors is depreciated and VTE has dropped it.
Switch to gtk_widget_override_background_color.

gtk_widget_modify_bg has been depreciated, as it uses GdkColor which is depreciated.

So use GdkRGBA and gtk_widget_override_background_color.
Drop uses of GTK_STOCK.

GTK is dropping GTK_STOCK, and so we're moving to just mnemonics with no icons.
Switch to VTE 2.91 from 2.90.

This involves a few changes:
vte_terminal_set_colors now takes GdkRGBA instead of GdkColor, so switch to a new 256colors.h.

vte_terminal_set_font_from_string no longer exists, so use pango_font_description_from_string and vte_terminal_set_font.

vte_terminal_fork_command_full has been replaced with vte_terminal_spawn_sync, which takes a new arugment that, for us, can just be NULL.

Use gdk_rgba_parse instead of gdk_color_parse to parse colors in config files.

Oh, and in the Makefile, bump the VTE version to 2.91 from 2.90.