Allow to compile against gtk3.

Allow to compile against gtk3.

Fixed not setting of completion colors.
Removed disabling of scrollbars.
This can be done for gtk2 via ~/.gtkrc-2.0:
    style "vimp-no-scrollbars" {
        GtkScrollbar::slider-width=0
        GtkScrollbar::trough-border=0
    }
    widget "vimp*" style "vimp-no-scrollbars"

and for gtk3 via XDG_CONFIG_HOME/gtk-3.0/gtk.css
    #vimp GtkScrollbar {
        -GtkRange-slider-width: 0;
        -GtkRange-trough-border: 0;
    }