Added resource classses and a missing converter.

Harmonia.WinUI/Resources/Converters.xaml

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="utf-8"?>
+<ResourceDictionary
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:converters="using:Harmonia.WinUI.Converters">
+
+    <converters:SecondsToStringConverter x:Key="SecondsToString" />
+    <converters:ArtistsToStringConverter x:Key="ArtistsToString" />
+    <converters:NullVisibilityConverter x:Key="NullVisibility" />
+    <converters:SongTitleConverter x:Key="SongTitle" />
+    <converters:DoubleToPercentConverter x:Key="DoubleToPercent" />
+    <converters:RepeatStateConverter x:Key="RepeatState" />
+    <converters:VolumeStateConverter x:Key="VolumeState" />
+
+</ResourceDictionary>