Allow to change toolbar font separately again

Summary:
The fonts KCM supports changing the toolbar font, but with the port to
QPA/Plasma 5 in aa16577f this stopped working and toolbars just used the
general font. This is fixed by setting QPlatformTheme's ToolButtonFont
to the toolbar font chosen. However, there are some caveats:

Default toolbar font size is 9, while the general font size is 10.
This would result in a sudden shrink in toolbar widths everywhere (or
worse for weird leftover settings users might have tried unsuccessfully).

In some places QToolButtons are used outside of toolbars, sometimes
right next to QPushButtons. As due to the bug both had the same font,
this might now lead to unexpected differences.

Therefore, to keep visual consistency with past Plasma 5 releases, let's
change the default toolbar font size to 10 and provide a kconf_update
script to copy the general font to the toolbar font once.

BUG: 358254
FIXED-IN: 5.11.0

Test Plan:
make install, run kconf_update: toolBarFont in kdeglobals gets updated
immediately

Change "Toolbar" font in "kcmshell5 fonts", start your favorite KDE
and Qt-only applications: toolbar font and/or font size changed
accordingly

Reviewers: Plasma, davidedmundson, fvogt

Reviewed By: Plasma, davidedmundson, fvogt

Subscribers: fvogt, davidedmundson, jensreuterberg, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D6697