Fixed runtime compilation error for Qt4

Hi All,

As sandeep from EDB was doing the setup for pgAdmin4 runtimes in CentOS 7
with Qt4 and found that there were some compilation errors.

Please find the attached patch file which will fix the below compilation
errors for Qt4.

- QtWebKitWidgets should only be included in Qt5. There is no such
header in Qt4.
- QUrl class does not have "url()" function in Qt4 so when we compile
the code in Qt4 it gives error saying "no such function url()". This is
fixed by adding common method ( "host()" ) available in both the version of
Qt.
- In QTcpSocket class, there is no "bind" method available in Qt4 so it
gives error during the compilation.
- "QThread::sleep(1)" is the public method in Qt5 while it is protected
in Qt4 so we can not use the same method in Qt4. We have written delay
function to replace this method.
- Removed "cache()" from Qt project file. Cache file is required by
qmake to read special setting specified other then project and qmake.conf
file, as we have not specified any special configuration other then project
file so we have removed this function.

We have tested in both the versions Qt4 and Qt5 and now it is compiling
without any errors.

Do review it and let us know in case of any comments/queries.

Thanks,
Neel Patel