Panel spacer that can center things

Summary:
* Make the panel spacer take a lot more space when is in expanding mode, so much
to "win" against the taskbar and collapse it to its minimum size (seems the
most expected behavior after some discussions in vdg channel)
* make it paint a background when in edit mode
* when two spacers are present in the panel, they try to center all the content
that is in between them (if possible), so that is always at the center of the panel,
even if there is more content on one side rather than the other

Test Plan:
{F8110981}
{F8112648}

Reviewers: #plasma, #vdg, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, davidre, broulik, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27481