AbstractPageHeader: reduce minimum title left padding
The current minimum left padding for the title text (when there
are no other UI elements, like navigation tools) is set gridUnit,
which can be a huge amount (18px on default settings) and is not
comparable with that of the QtWidgets equivalent.
Reduce this value to largeSpacing, which also aligns better with
the contents within the page.
This is also meant to help with plasma/systemsettings!144 (merged)
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Edited by Ismael Asensio