Move qucssetting to singleton

This is a big modification.
The QucsSettings is actually quite annoying, and is not a good implementation.
Here I move setting to a singleton class, all the property is saved in QVariant.
You can use:
```
set(section, key, value)
get(section, key)
testSet(section, key)
```
to access the data.
Since the data stored in QVariant, so after you call get, you have to cast it to the datatype you want, like:
```
get("a", "b").toString()
```
Setting will be initialized in main.cpp and other require setting just include setting.h, every time you call `set`, the change will write to file directly, so savesetting at end is no more needed.
******
Since this is a large modification, I suggest to make it a branch for a while and accept comment and patch from everyone.