[kdecorations] Add a ::palette() to the decoration API

The ::palette() in KDecoration and KCommonDecoration returns the
QPalette the decoration should use for the decorated window. The
call delegates into the bridge and KWin core might provide a special
QPalette for a given Client depending on the _KDE_NET_WM_COLOR_SCHEME
property.

bridge.cpp

@@ -65,6 +65,7 @@ BRIDGE_HELPER(void, maximize, MaximizeMode m, m,)
 BRIDGE_HELPER(void, minimize, , ,)
 BRIDGE_HELPER(void, showContextHelp, , ,)
 BRIDGE_HELPER(void, setDesktop, int desktop, desktop,)
+BRIDGE_HELPER(QPalette, palette, , , const)
 
 bool Bridge::isActive() const
 {

bridge.h

@@ -81,6 +81,7 @@ public:
     virtual QRect transparentRect() const override;
 
     virtual void update(const QRegion &region) override;
+    virtual QPalette palette() const override;
 
     // Window tabbing
     virtual QString caption(int idx) const override;

kcmkwin/kwindecoration/preview.cpp

@@ -552,6 +552,11 @@ KDecoration::WindowOperation KDecorationPreviewBridge::buttonToWindowOperation(Q
     return KDecoration::NoOp;
 }
 
+QPalette KDecorationPreviewBridge::palette() const
+{
+    return QApplication::palette();
+}
+
 KDecorationPreviewOptions::KDecorationPreviewOptions()
 {
     customBorderSize = BordersCount; // invalid

kcmkwin/kwindecoration/preview.h

@@ -150,6 +150,7 @@ public:
     virtual QRect transparentRect() const override;
 
     virtual void update(const QRegion &region) override;
+    virtual QPalette palette() const override;
 
     // Window tabbing
     virtual QString caption(int idx) const override;

libkdecorations/kcommondecoration.cpp

@@ -1056,6 +1056,11 @@ void KCommonDecoration::update(const QRegion &region)
     d->wrapper->update(region);
 }
 
+QPalette KCommonDecoration::palette() const
+{
+    return d->wrapper->palette();
+}
+
 class KCommonDecorationButtonPrivate
 {
 public:

libkdecorations/kcommondecoration.h

@@ -235,6 +235,8 @@ public:
      */
     QRect transparentRect() const;
 
+    QPalette palette() const;
+
 public:
     /**
      * Handles widget and layout creation, call the base implementation when subclassing this member.

libkdecorations/kdecoration.cpp

@@ -453,6 +453,11 @@ bool KDecoration::compositingActive() const
     return d->bridge->compositingActive();
 }
 
+QPalette KDecoration::palette() const
+{
+    return d->bridge->palette();
+}
+
 void KDecoration::padding(int &left, int &right, int &top, int &bottom) const
 {
     left = right = top = bottom = 0;

libkdecorations/kdecoration.h

@@ -823,6 +823,11 @@ public:
      **/
     bool isAlphaEnabled() const;
 
+    /**
+     * The recommended palette for the decoration. This can be different per decorated client.
+     */
+    QPalette palette() const;
+
     // requests to decoration
 
     /**

libkdecorations/kdecorationbridge.h

@@ -88,6 +88,7 @@ public:
     virtual QRect transparentRect() const = 0;
 
     virtual void update(const QRegion &region) = 0;
+    virtual QPalette palette() const = 0;
 
     // Window tabbing
     virtual QString caption(int idx) const = 0;