When resizing the Qt Creator main window to a smaller size, at some
point the mode tab icons will shrink to fit all tabs into the window.
The icon size will if necessary shrink to tiny sizes below 8px,
which is pointless.

Let's add some code that hides the icons when a tab becomes smaller
than a certain size.

Task-Number: QTCREATORBUG-7879

Change-Id: I590379a2ec52a8a86e998c08b29d54aca800e542
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@nokia.com>