VisualBuilder: Hook up everything needed for widget property editing.

It's now possible to edit widget properties inline in the properties window.
We're currently relying on the basic GVariant conversion functions to do
all the "parsing" but that's not gonna be good enough.

LibGUI/GProgressBar.h

@@ -8,6 +8,8 @@ public:
     virtual ~GProgressBar() override;
 
     void set_range(int min, int max);
+    void set_min(int min) { set_range(min, max()); }
+    void set_max(int max) { set_range(min(), max); }
     void set_value(int);
 
     int value() const { return m_value; }

LibGUI/GScrollBar.h

@@ -16,6 +16,8 @@ public:
     int step() const { return m_step; }
     int big_step() const { return m_big_step; }
 
+    void set_min(int min) { set_range(min, max()); }
+    void set_max(int max) { set_range(min(), max); }
     void set_range(int min, int max);
     void set_value(int value);
     void set_step(int step) { m_step = step; }

LibGUI/GSpinBox.h

@@ -15,6 +15,8 @@ public:
 
     int min() const { return m_min; }
     int max() const { return m_max; }
+    void set_min(int min) { set_range(min, max()); }
+    void set_max(int max) { set_range(min(), max); }
     void set_range(int min, int max);
 
     Function<void(int value)> on_change;

LibGUI/GVariant.cpp

@@ -101,6 +101,17 @@ GVariant& GVariant::operator=(const GVariant& other)
     return *this;
 }
 
+GVariant& GVariant::operator=(GVariant&& other)
+{
+    if (&other == this)
+        return *this;
+    // FIXME: Move, not copy!
+    clear();
+    copy_from(other);
+    other.clear();
+    return *this;
+}
+
 GVariant::GVariant(const GVariant& other)
 {
     copy_from(other);

LibGUI/GVariant.h

@@ -22,7 +22,7 @@ public:
     GVariant& operator=(const GVariant&);
 
     GVariant(GVariant&&) = delete;
-    GVariant& operator=(GVariant&&) = delete;
+    GVariant& operator=(GVariant&&);
 
     void clear();
     ~GVariant();
@@ -91,6 +91,13 @@ public:
             return as_bool() ? 1 : 0;
         if (is_float())
             return (int)as_float();
+        if (is_string()) {
+            bool ok;
+            int value = as_string().to_int(ok);
+            if (!ok)
+                return 0;
+            return value;
+        }
         return 0;
     }