QtVirtualKeyboard needs to use declarative type registration

QtVirtualKeyboard needs to use declarative type registration and stop using qmlplugindump to generate qmltypes files in order for it to be compatible with qml tooling (and the compiler).

Some pointers:

• Make the plugin (src/import) optional like done in this change OR merge the plugin part into the actual library
  • You might need to turn the C++ types into an extra .impl module for this to work
• Create a proper module structure since you can't register qml files anymore and install all your qml files instead of just bundling them with the library