Use Qml scripting

Requires Qt 5.8 and up because of requiring
`QJSEngine::newQMetaObject()` to register new constructors (ByteArray(),
File(), ...).

Syntax errors can no longer be reported in command edit widget.

New behavior: ByteArray is implicitly converted to string when needed,
e.g. these expressions return true:

    ByteArray("abc") == "abc"
    ByteArray("ab") + "c" == "abc"

Fixes #329
Signed-off-by: Lukas Holecek <hluk@email.cz>