Counter expressions (#3534)

* Add peglib

* - Add expression engine
- Take an expression when setting a counter

* Shift + Click = Middleclick for counters

* minor cleanup for clangify

Signed-off-by: Zach Halpern <ZaHalpern+github@gmail.com>

* Added tip entry

common/expression.h

@@ -0,0 +1,28 @@
+#ifndef EXPRESSION_H
+#define EXPRESSION_H
+
+#include <QMap>
+#include <QString>
+#include <functional>
+
+namespace peg
+{
+template <typename Annotation> struct AstBase;
+struct EmptyType;
+typedef AstBase<EmptyType> Ast;
+} // namespace peg
+
+class Expression
+{
+public:
+    double value;
+
+    explicit Expression(double initial = 0);
+    double parse(const QString &expr);
+
+private:
+    double eval(const peg::Ast &ast);
+    QMap<QString, std::function<double(double)>> fns;
+};
+
+#endif