[gnome-taquin] Bump Gtk required version to 3.20.

[Arnaud B. <arnaudb src gnome org>]

commit 654b728da87346ddcb0a084dca40eba0d5db0545
Author: Arnaud Bonatti <arnaud bonatti gmail com>
Date:   Thu Jan 17 16:26:45 2019 +0100

    Bump Gtk required version to 3.20.
    
    Clean a bit CSS, you should have updated.

 data/taquin.css | 16 ----------------
 meson.build     |  4 ++--
 2 files changed, 2 insertions(+), 18 deletions(-)
---
diff --git a/data/taquin.css b/data/taquin.css
index 0d9a465..9648d9f 100644
--- a/data/taquin.css
+++ b/data/taquin.css
@@ -17,53 +17,37 @@
 
 @import url("base-window.css");
 
-/* GtkXyz is pre-3.19.n */
-
-GtkModelButton.radio-like-toggle#radio-fifteen,
 button.radio-like-toggle#radio-fifteen {
     background-image: url("resource://org/gnome/Taquin/images/15-Puzzle.png");
 }
-GtkModelButton.radio-like-toggle#radio-sixteen,
 button.radio-like-toggle#radio-sixteen {
     background-image: url("resource://org/gnome/Taquin/images/16-Puzzle.png");
 }
 
-GtkModelButton.radio-like-toggle,
 button.radio-like-toggle {
     background-repeat: no-repeat;
     background-position: center 25px;
     color: @theme_fg_color;
     background-color: transparent;
 }
-GtkModelButton.radio-like-toggle GtkLabel,
 button.radio-like-toggle label {
     padding-bottom: 165px;
 }
-GtkModelButton.radio-like-toggle:backdrop,
 button.radio-like-toggle:backdrop {
     color: @theme_unfocused_fg_color;
 }
 
-GtkModelButton.radio-like-toggle,
-GtkMenuButton.menubutton-like-combo {
-    border-radius:15px;
-    -gtk-outline-radius:13px;
-}
 button.radio-like-toggle,
 button.menubutton-like-combo {
     border-radius:15px;
     -gtk-outline-radius:13px;
 }
 
-GtkMenuButton.menubutton-like-combo:checked,
-GtkMenuButton.menubutton-like-combo:hover,
 button.menubutton-like-combo:checked,
 button.menubutton-like-combo:hover {
     color:#2e3436; /* gtk-contained.css’ theme_fg_color */
     background:-gtk-gradient (radial, center center, 0, center center, 0.5, from(rgba (255, 255, 255, 1)), 
to(rgba (255, 255, 255, 0)));
 }
-GtkMenuButton.menubutton-like-combo:checked:backdrop,
-GtkMenuButton.menubutton-like-combo:hover:backdrop,
 button.menubutton-like-combo:checked:backdrop,
 button.menubutton-like-combo:hover:backdrop {
     color:@theme_unfocused_fg_color;
diff --git a/meson.build b/meson.build
index 924dee5..dfb63dc 100644
--- a/meson.build
+++ b/meson.build
@@ -8,7 +8,7 @@ i18n = import('i18n')
 python3 = import('python3')
 
 glib_dependency = dependency('glib-2.0', version: '>= 2.40.0')
-gtk_dependency = dependency('gtk+-3.0', version: '>= 3.15.0')
+gtk_dependency = dependency('gtk+-3.0', version: '>= 3.20.0')
 rsvg_dependency = dependency('librsvg-2.0', version: '>= 2.32.0')
 canberra_depenency = dependency('libcanberra')
 canberra_gtk3_depenency = dependency('libcanberra-gtk3', version: '>= 0.26')
@@ -41,4 +41,4 @@ subdir('src')
 
 meson.add_install_script(python3.find_python().path(), join_paths(meson.source_root(), 
                                                                   'build-aux', 
-                                                                  'meson_post_install.py'))
\ No newline at end of file
+                                                                  'meson_post_install.py'))