Fix Qt deprecation warns for setting tab

Fix Qt deprecation warns for setting tab

Fix warnings coming from deprecations of QTextEdit::tabStopWidth()
and QFontMetrics::width(). Regarding tabStopWidth(), the ChangeLog
states the following [1]:

  Introduced tabStopDistance property in QTextOption, QTextEdit and
  QPlainTextEdit as replacement for the inconsistently named tabStop and
  tabStopWidth properties. QTextOption::tabStop, QTextEdit::tabStopWidth and
  QPlainTextEdit::tabStopWidth are now deprecated.

Note that QFontMetrics::horizontalAdvance() is what we want here, as
opposed to QFontMetrics::boundingRect(), because we want to know
where to draw the next character after the tab.

[1] https://code.qt.io/cgit/qt/qtbase.git/tree/dist/changes-5.10.0/?h=v5.10.0