Progress on various property editor features

* Added support for editing top-level custom properties (excluding support
  for classes and enums for now).

* Added support for collapsing the property groups, based on a new
  GroupProperty which creates nested VariantEditor instances.

* Removed QVariant based getter/setter from Property, since it doesn't
  appear to be useful.

* Added Property::DisplayMode, which is used to tell apart group
  properties and separators from regular properties.

* VariantEditor is no longer a scroll area itself, to enable nesting.

src/tiled/fileedit.cpp

@@ -45,7 +45,7 @@ FileEdit::FileEdit(QWidget *parent)
     mOkTextColor = mLineEdit->palette().color(QPalette::Active, QPalette::Text);
 
     QToolButton *button = new QToolButton(this);
-    button->setText(QLatin1String("..."));
+    button->setText(QStringLiteral("…"));
     button->setAutoRaise(true);
     button->setToolTip(tr("Choose"));
     layout->addWidget(mLineEdit);