Add new component for the greyed out labels in Item Delegates.

Summary:
Plasma code is full of Label {opacity: 0.6}. We shouldn't be hardcoding
style into a UI, especially with magic numbers.

This adds a new component that does the same thing but will allow
plasmoids to write more semantic code.

Reviewers: #plasma, hein

Reviewed By: #plasma, hein

Subscribers: hein, sebas, broulik, plasma-devel, #frameworks

Tags: #plasma, #frameworks

Differential Revision: https://phabricator.kde.org/D4831

src/declarativeimports/plasmaextracomponents/qml/DescriptiveLabel.qml

@@ -0,0 +1,55 @@
+/*
+*   Copyright 2012 by Sebastian Kügler <sebas@kde.org>
+*
+*   This program is free software; you can redistribute it and/or modify
+*   it under the terms of the GNU Library General Public License as
+*   published by the Free Software Foundation; either version 2, or
+*   (at your option) any later version.
+*
+*   This program is distributed in the hope that it will be useful,
+*   but WITHOUT ANY WARRANTY; without even the implied warranty of
+*   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+*   GNU Library General Public License for more details
+*
+*   You should have received a copy of the GNU Library General Public
+*   License along with this program; if not, write to the
+*   Free Software Foundation, Inc.,
+*   51 Franklin Street, Fifth Floor, Boston, MA  2.010-1301, USA.
+*/
+
+import QtQuick 2.0
+import org.kde.plasma.components 2.0
+
+/**
+ * This is a descriptive label which uses the plasma theme. The characteristics of
+ * the text will be automatically set according to the plasma theme. Use this
+ * components for less important additional data to show in a user interface.
+ *
+ * Example usage:
+ * @code
+ * import org.kde.plasma.extras 2.0 as PlasmaExtras
+ * [...]
+ * Column {
+ *     PlasmaComponents.Label { text: "Firefox" }
+ *     PlasmaExtras.DescriptiveLabel { text: "Web Browser"}
+ *   [...]
+ * }
+ * @endcode
+ *
+ * See PlasmaComponents Label and primitive QML Text element API for additional
+ * properties, methods and signals.
+ *
+ * @inherits org::kde::plasma::components::Label
+ */
+Label {
+    id: root
+
+    /*
+     * If a user can interact with this item, for example in a ListView delegate, this
+     * property should be set to true when the label is being interacted with.
+     * The default is false
+     */
+    property bool active: false
+
+    opacity: active ? 0.8 : 0.6
+}

src/declarativeimports/plasmaextracomponents/qml/qmldir

@@ -8,6 +8,7 @@ Paragraph 2.0 Paragraph.qml
 PageRow 2.0 PageRow.qml
 ScrollArea 2.0 ScrollArea.qml
 Title 2.0 Title.qml
+DescriptiveLabel 2.0 DescriptiveLabel.qml
 
 ActivateAnimation 2.0 animations/ActivateAnimation.qml
 AppearAnimation 2.0 animations/AppearAnimation.qml