Instead of copyign a pre-existing .db shipped with QGIS, create
a new .db and import an .xml file containing our default symbols.
On the UX front, the main benefit is being able to ship with
pre-defined tags and favorite flags for the default symbols.
On the developer side, it means we get rid of the requirement
to maintain both an .xml and a binary .db in the source tree.
We also say aurevoir to the symbol_xml2db.py script and the
need to update the (obsolete) script every time we add a new
feature to QgsStyle.
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