editor: use QVector rather than std::vector

Using the same implementation on all platforms leads to less
platform-surprises.
editor: use QByteArray for WebSocket buffer

Instead of using std::string, let's use QByteArray. While we're at
it, change some size_t's to int to avoid casts.
editor: use qMin/qMax/qBound
editor: use QByteArrays for WebSocket upgrade
editor: QString for track-names
editor: fixup a cast
editor: use QMap for clientTracks
editor: use Q_ASSERT instead of assert
editor: remove needless includes

Now we only depend on Qt for the editor. No standard-lib includes
at all.
editor: add missing include-guard
editor: prefer standard header-guards

Pragmas are language extensions, so let's not depend on them. Use
"normal" header-guards instead.