Show Japanese text and close the gap in the top bar

Godot's built-in font has no Japanese, so kanji in song names showed as
boxes. The theme now sets Godot's own Open Sans SemiBold as its default
font, with Noto Sans JP Medium as fallback. The Noto file's line metrics
are set to Open Sans's, so line heights stay the same and every other
screen is pixel-identical; fonts/README.md explains how to redo it.

Search and Sync move right to where the Statistics button was.

themes/main_theme.tres

@@ -1,5 +1,7 @@
 [gd_resource type="Theme" format=3]
 
+[ext_resource type="FontFile" path="res://fonts/OpenSans-SemiBold.woff2" id="1_default_font"]
+
 [sub_resource type="StyleBoxFlat" id="StyleBoxFlat_vxwtt"]
 content_margin_left = 4.0
 content_margin_top = 4.0
@@ -748,6 +750,7 @@ corner_radius_bottom_right = 6
 corner_radius_bottom_left = 6
 
 [resource]
+default_font = ExtResource("1_default_font")
 Button/colors/font_color = Color(0.875, 0.875, 0.875, 1)
 Button/colors/font_disabled_color = Color(0.875, 0.875, 0.875, 0.5)
 Button/colors/font_focus_color = Color(0.95, 0.95, 0.95, 1)