KColorScheme: default to application scheme if set by KColorSchemeManager

KColorScheme: default to application scheme if set by KColorSchemeManager

KColorSchemeManager::activateScheme() sets a custom path for the
application's color scheme, with
`qApp->setProperty("KDE_COLOR_SCHEME_PATH", index.data(Qt::UserRole));`

Currently, the KColorScheme() and KStatefulBrush() constructors will
 ignore this and use only the system color scheme, unless an
 application-specific config is explicitly loaded and passed in by the
 caller.
This is problematic, because most callers assume that the default is to
 match the *application* scheme - usually this is equivalent, but it
 differs when KColorSchemeManager is used.

For example, when the application of a KTextEditor widget or
 KonsolePart has an opposite color scheme to the system, the Find bars
 are unreadable.
BUG: 373764

This patch makes KColorScheme() match the application scheme by default
 when this differs from the system scheme, which seems preferable to
 adding the same code in hundreds of callers.

Differential Revision: https://phabricator.kde.org/D4637

Gbp-Pq: Name KColorScheme-default-to-application-scheme-if-set-by-KCol.patch