[Gtk] Fix letters mapping when not holding Shift

src/Gtk/Perspex.Gtk/Input/GtkKeyboardDevice.cs

@@ -83,6 +83,32 @@ namespace Perspex.Gtk
             { Gdk.Key.X, Key.X },
             { Gdk.Key.Y, Key.Y },
             { Gdk.Key.Z, Key.Z },
+            { Gdk.Key.a, Key.A },
+            { Gdk.Key.b, Key.B },
+            { Gdk.Key.c, Key.C },
+            { Gdk.Key.d, Key.D },
+            { Gdk.Key.e, Key.E },
+            { Gdk.Key.f, Key.F },
+            { Gdk.Key.g, Key.G },
+            { Gdk.Key.h, Key.H },
+            { Gdk.Key.i, Key.I },
+            { Gdk.Key.j, Key.J },
+            { Gdk.Key.k, Key.K },
+            { Gdk.Key.l, Key.L },
+            { Gdk.Key.m, Key.M },
+            { Gdk.Key.n, Key.N },
+            { Gdk.Key.o, Key.O },
+            { Gdk.Key.p, Key.P },
+            { Gdk.Key.q, Key.Q },
+            { Gdk.Key.r, Key.R },
+            { Gdk.Key.s, Key.S },
+            { Gdk.Key.t, Key.T },
+            { Gdk.Key.u, Key.U },
+            { Gdk.Key.v, Key.V },
+            { Gdk.Key.w, Key.W },
+            { Gdk.Key.x, Key.X },
+            { Gdk.Key.y, Key.Y },
+            { Gdk.Key.z, Key.Z },
             //{ Gdk.Key.?, Key.LWin }
             //{ Gdk.Key.?, Key.RWin }
             //{ Gdk.Key.?, Key.Apps }