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/README.md

@@ -13,6 +13,9 @@ checks that it stays that way.
    sizes in the theme editor. Keep every style's name.
 3. Point Project Settings → GUI → Theme → Custom at the copy.
 
+The **font** is the theme's default font, Godot's own Open Sans with Noto Sans JP for
+Japanese text; see `fonts/README.md`.
+
 Icons (play, restart, add player, the slider and switch knobs) are not part of the theme;
 they come from the image files in `icons/` and Godot's built-in theme.
 

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)