Libreoffice Theme Part 0: Read correct colors from qt based VCL_PLUGINS

See `QtFrame::UpdateSettings()`, where `StyleSettings::WindowColor` is
set to aBase (QPalette::Base) and `StyleSettings::DialogColor` is set to
WindowColor in a call to `StyleSettings::BatchSetBackgrounds`.

The issue is that without this patch in place, "GetWindowColor()" doesn't
return the color of the window, instead it returns the "base color" from
QPalette.

It had some unintendedd consequences like the "Animation Panel" in the
Impress's Animations sidebar not being painted with the correct color,
as it was expecting the old WindowColor. This patch also fixes that by
changing WindowColor to FieldColor in `SalInstanceWidget::set_stack_background()`

https://gerrit.libreoffice.org/c/core/+/168901/comment/0e5201bd_2cae07b9/

Change-Id: I36f8c5efd8c3e87ded19a1efc00ed9c69a7fccf1
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/171916
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

sc/source/ui/dbgui/csvruler.cxx

@@ -156,7 +156,7 @@ void ScCsvRuler::InitColors()
 {
     const StyleSettings& rSett = Application::GetSettings().GetStyleSettings();
     maBackColor = rSett.GetFaceColor();
-    maActiveColor = rSett.GetWindowColor();
+    maActiveColor = rSett.GetFieldColor();
     maTextColor = rSett.GetLabelTextColor();
     maSplitColor = maBackColor.IsDark() ? maTextColor : COL_LIGHTRED;
     InvalidateGfx();

vcl/qt5/QtFrame.cxx

@@ -1130,7 +1130,6 @@ void QtFrame::UpdateSettings(AllSettings& rSettings)
 
     // Base
     style.SetFieldColor(aBase);
-    style.SetWindowColor(aBase);
     style.SetActiveTabColor(aBase);
     style.SetListBoxWindowBackgroundColor(aBase);
     style.SetAlternatingRowColor(toColor(pal.color(QPalette::Active, QPalette::AlternateBase)));
@@ -1163,6 +1162,7 @@ void QtFrame::UpdateSettings(AllSettings& rSettings)
     // Background
     style.BatchSetBackgrounds(aBack);
     style.SetInactiveTabColor(aBack);
+    style.SetWindowColor(aBack);
 
     // Workspace
     style.SetWorkspaceColor(aMid);

vcl/source/app/salvtables.cxx

@@ -715,7 +715,7 @@ void SalInstanceWidget::get_property_tree(tools::JsonWriter& rJsonWriter)
 
 void SalInstanceWidget::set_stack_background()
 {
-    set_background(m_xWidget->GetSettings().GetStyleSettings().GetWindowColor());
+    set_background(m_xWidget->GetSettings().GetStyleSettings().GetFieldColor());
 }
 
 void SalInstanceWidget::set_title_background()