Port and update the Image Gallery plugin

kDebug(), qDebug() -> qCDebug() with logging category
No need to link with KDELibs4Support
Convert layouts to use QFormLayout
KIntNumInput -> QSpinBox
Use Qt5 signal connection syntax
Replace deprecated "What's This" with tool tips
KToolInvocation::invokeBrowser() -> QDesktopServices::openUrl()
KLocale::formatDate() -> QLocale
KImageIO::pattern() -> QImageReader/QMimeDatabase
KIO::NetAccess::file_copy() -> QFile::copy() - files are known to be local