Add helper classes to support strings containing variables

* VariableResolvingString will accept strings potentially containing
   variables as inputs and will automatically resolve any variables when
   converting to std::string
 * VariableTextEdit is a simple wrapper around ResizingPlainTextEdit to
   enable working with VariableResolvingString

src/utils/variable-text-edit.cpp

@@ -0,0 +1,18 @@
+#include "variable-text-edit.hpp"
+#include "switcher-data-structs.hpp"
+
+VariableTextEdit::VariableTextEdit(QWidget *parent)
+	: ResizingPlainTextEdit(parent)
+{
+}
+
+void VariableTextEdit::setPlainText(const QString &string)
+{
+	QPlainTextEdit::setPlainText(string);
+}
+
+void VariableTextEdit::setPlainText(const VariableResolvingString &string)
+{
+	QPlainTextEdit::setPlainText(
+		QString::fromStdString(string.UnresolvedValue()));
+}