LibVT+Terminal: Add color scheme support

This commit introduces color scheme support to Terminal. These are found
in `/res/terminal_colors` and the default color scheme can be set in
`~/.config/Terminal.ini`. Furthermore, a combo box is added for
setting the color scheme at runtime.

The previously used default color scheme has been added to
`/res/terminal-colors/Default.ini`.

To make the implementation more compatible with other color schemes,
`TerminalWidget` now supports overriding the default foreground and
background colors.

Userland/Applications/Terminal/TerminalSettingsWindow.gml

@@ -60,4 +60,17 @@
             orientation: "Horizontal"
         }
     }
+
+    @GUI::GroupBox {
+        title: "Color scheme"
+        shrink_to_fit: true
+
+        layout: @GUI::VerticalBoxLayout {
+            margins: [6, 16, 6, 6]
+        }
+
+        @GUI::ComboBox {
+            name: "color_scheme_combo"
+        }
+    }
 }