STYLE: Refactor function onShowToolButtonTextToggled re-using existing code

See #2164

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22372 3bd1e089-480b-0410-8dfb-8563597acbee
BUG: Apply "ShowToolButtonText" settings to mainwindows on restart

While the slot qSlicerSettingsStylesPanel::onShowToolButtonTextToggled()
is properly called when the settings is registered at startup, it failed
to disable/enable the toolButton property because the mainwindow is not
instantiated.

This commit ensure this is the case in the main windows readSettings()
function.

Fixes #2164

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22373 3bd1e089-480b-0410-8dfb-8563597acbee