Reduce width of the plan playback panel

[patmarion]

There's a bug that causes the panel to resize itself to an unnecessarily large width for an instant, this breaks the main window sizing which is very annoying if you don't have a high resolution display.

After fixing that bug, if the min width of the panel is still too large, there are some options:

• use two rows to layout the widgets above the slider, currently they are arranged in a single row
• instead of two rows, use a "flow" layout, similar to a responsive web site, http://doc.qt.io/qt-5/qtwidgets-layouts-flowlayout-example.html