Fix width issues when un-maximizing Dolphin

Before this commit un-maximizing or to be more precise instantly
changing the size of the window by a large amount could potentially
change the width of the sidebars, the viewContainers and the window
itself to unexpected/undesired widths.

This happened because the spacing calculation is triggered when the
primary ViewContainer is resized but at this point in time some of
the other widgets, especially the secondary ViewContainer and the
navigatorsWidget have generally not been resized yet. Therefore the
width and spacing calculations are based on partly updated and
partly outdated values leading to wrong results.

This commit makes it so calculation of spacings is delayed until
all widths have been updated.

Yes, spacing probably should not have the power to resize the
window but unfortunately the spacing can not be set to be less
forceful when taking space because otherwise the UrlNavigators
will take all space they can get with their

BUG: 430521
FIXED-IN: 21.08
1 job for !250 with fix_resizing_problems in 4 minutes and 55 seconds (queued for 8 seconds)
latest detached
Status Name Job ID Coverage
passed linux kf5-qt5 qt5.15 #121058