qtbase: Support custom app-specific standard paths

Additionally to environment variables the placeholder ::APPNAME:: is
replaced with the application name in the customized standard locations.
Moreover, the names of enum values are now resolved using QMetaEnum to
fix a wrong mapping of some enum values to their enum name and to avoid
such problems in the future.

(cherry picked from commit 4faba785dd332848741540c9f02297e97ec5f714)

• GnuPG-bug-id: T7717