ApplicationHeader: Make top toolbars the same height/spacing as QWidget ones

Marco Martinrequested to merge
work/mart/consisterHeaderHeight into master

For consistency, add padding around AbstractApplicationHeader to
smallSpacing, this makes top toolbars exactly as tall as the ones in
qwidget applications, which have a bit more whitespace around making
them look a bit more elegant.

needs qqc2-desktop-style!492 (merged)

Edited by Marco Martin

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