Optimize ShaderEffect construction

ShaderEffect is a relatively heavy compnent to construct compared to other built in components like Image and Rectangle. This isn't an issue when using ShaderEffect for large, infrequent effects, but can become an issue when e.g. using it on every element in a GridView.

Some ideas:

• See if lookThroughShaderCode can be applied per-unique-shader, rather than per-ShaderEffect-instance.
• String-based connects; QSignalMapper; string-based property lookups, etc are all relatively heavy weight. See if they can be replaced by lighter weight alternatives.