Fix Qt deprecation warn for getContentsMargins()

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