[LyX/master] Add +/- buttons to zoom slider

[Pavel Sanda]

On Tue, Mar 09, 2021 at 08:37:28AM +0100, Juergen Spitzmueller wrote:
> commit ebb7ee7d5f8a5aea29142ffc794617e34ee13fb8
> Author: Juergen Spitzmueller <spitz at lyx.org>
> Date:   Tue Mar 9 08:42:27 2021 +0100
> 
>     Add +/- buttons to zoom slider
> ---
>  src/frontends/qt/GuiView.cpp |   35 +++++++++++++++++++++++++++++++++++
>  src/frontends/qt/GuiView.h   |    9 +++++++++
>  2 files changed, 44 insertions(+), 0 deletions(-)

- Plus sign is now enabled even when no document is loaded (and causes crash
  if triggered in such scenario).
- Both signs are not verticaly aligned with the slider and thus are somewhat 
  aesthetically disturbing, not sure how to fix that given the variety of
  architectures/themes/fonts...

Pavel