[sailfish-browser] Use silica invoker instead of qt5 invoker

At some point silica invoker caused issues to
rendering (don't know why) but those are now history with
threaded rendering.

With V4 precompilation of QML components has become even more important.
This improves sailfish-browser startup time quite significantly
as all Silica components will be compiled by booster.
Merge pull request #188 from rainemak/silicaInvoker

[sailfish-browser] Use silica invoker instead of qt5 invoker