load language setting dynamically

This is just a demo branch, DO NOT MERGE THIS.
You can try change language in setting dialog, then change a components category. You will find that components' name change to language you choose. 
However, the names of the menu and action don't change, since we have to call setText(tr("..."))
after language being set. That is, we have to write qucs_action.cpp like this:
```
function init_action(){
  QAction file = new ......
  .....
}

function setup_text() {
  file->setText...
  ....
```
so that we can call setup_text() after language changed. This will be a hard work, maybe we can do that during QScrollView change?