Qt5

>reopen for https://github.com/haiwen/seafile-client/pull/151

ATTENTION: the PR remains for test purposes.

**PLEASE NOT MERGE THIS**

least requirement: Qt4 >= 4.8.0 Qt5 >= 5.00 
> For Qt5, if you are using clang>=3.4, Qt5>=5.1.1 is required

recommended requirement: Qt5.3.1