window: Remove use of GTK modifier styles

gtk_widget_modify_font(), gtk_widget_modify_base(), and modifier styles
are deprecated. This replaces them with GtkStyleContext and
gtk_widget_override_* (which itself should later be replaced with CSS.)
grid: Don't use gtk_entry_set_inner_border()

It is ignored nowadays, and was causing line input in grid windows to
get out of whack. Instead, add some CSS to GtkEntries before sticking
them into grid windows.

The style class ".flat" works well for this, but is only available in
GTK 3.14 or later.
grid: Style line input box background with CSS

As long as we are adding CSS to the GtkEntry that does line input in
grid windows anyway, use it to override the background color instead of
gtk_widget_override_background_color().
grid: No need to override the font

The line input GtkEntry now inherits its font from the text grid window,
so no need to set it explicitly with gtk_widget_override_font().