[LyX/master] Fix Qt deprecation warn for getContentsMargins()

[Scott Kostyshak]

commit 1c78b563ff47cafbd2c1622342ca3c639e4738ca
Author: Scott Kostyshak <skostysh at lyx.org>
Date:   Fri Mar 6 09:07:31 2020 -0500

    Fix Qt deprecation warn for getContentsMargins()
    
    Fixes the following warning:
    
      error: ‘void QWidget::getContentsMargins(int*, int*, int*, int*) const’ is deprecated: use contentsMargins() [-Werror=deprecated-declarations]
    
    The new code also has advantages in that 'l' can be set to const and
    it is more readable because it is clear that only 'l' is used.
    
    QWidget::getContentsMargins() was deprecated in 5.14.0 [1]. and
    QWidget::contentsMargins() is available since 4.6 [2].
    
    [1] https://code.qt.io/cgit/qt/qtbase.git/tree/dist/changes-5.14.0/?h=v5.14.0
    [2] https://doc.qt.io/archives/qt-4.8/qlayout.html#contentsMargins
---
 src/frontends/qt/GuiView.cpp |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/frontends/qt/GuiView.cpp b/src/frontends/qt/GuiView.cpp
index 6890c16..c7a1bd9 100644
--- a/src/frontends/qt/GuiView.cpp
+++ b/src/frontends/qt/GuiView.cpp
@@ -4604,8 +4604,8 @@ bool GuiView::lfunUiToggle(string const & ui_component)
 		menuBar()->setVisible(!menuBar()->isVisible());
 	} else
 	if (ui_component == "frame") {
-		int l, t, r, b;
-		getContentsMargins(&l, &t, &r, &b);
+		int const l = contentsMargins().left();
+
 		//are the frames in default state?
 		d.current_work_area_->setFrameStyle(QFrame::NoFrame);
 		if (l == 0) {