Improve Qtwebkit memory usage

[raelgc]

After @stieg and few others reported the memory usage problem, I started to face the same after upgrade the webkit package in Ubuntu.

I found that it appears to be a memory leak in webkit: several users complaining about it.

After some research, I found that I can reduce the webkit memory usage with the following:

• share the same networkAcessManager
• call clearMemoryCaches