Remove spacers in non-LargeSideBar modes for FancyTabWidget

[amhndu]

To maintain behavior with the older FancyTabWidget, spacers are now
removed and stored when switching to a non-LargeSideBar mode.
And subsequently restored when switching back to LargeSideBar mode.

Fixes #6163

[jonaski]

Is there any reason why this is done on qt5 and not master?
You should make the pull requests on master unless it's a specific fix only to Qt 5, otherwise it makes it harder to merge.

[amhndu]

Thanks, I wasn't really aware. I thought qt5 is the main development branch

[amhndu]

@jonaski A different PR of mine was merged to qt5 similarly, should I make a new PR cherry-picking that onto master ? (and similarly for this)

[jonaski]

I'm not the project owner or maintainer here. But I suggest that yes.
Whats done until now is that we create PRs mainly on master and then we merge master into qt 5.
As long as they want to still keep Qt 4 support and have that in master.

[ghost]

I wonder if there's a reason to keep master on Qt 4.