Add a mode to center actions and omit the title when using a ToolBar style

Summary:
This patch implements an optional and off-by-default feature to center the actions and
omit the title when displaying the ToolBar style. This is useful for simple desktop apps
that don't need to display a title in the toolbar, where centered toolbuttons look
better than right-aligned toolbuttons.

FEATURE: 404203
CCBUG: 402948
FIXED-IN: 5.63

Test Plan:
Apply this patch to Kirigami then run Kamoso after applying D23847.

Before:

After:

Reviewers: Kirigami, VDG, mart, ndavis

Reviewed By: Kirigami, VDG, mart, ndavis

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D23845