[Patch][pgAdmin4][runtime]: RM#1375 - Migrate the runtime to QtWebEngine from QtWebKit

Hi,

Please find attached initial patch for migration of runtime to QtWebEngine
from QtWebkit.

*Observation:-*

When user click on already opened link, it will not set existing tab index
but always open in new tab.
e.g. pgAdmin4 website is already open in new tab and user again open the
same website then it will not set existing tab index but always open in new
tab.

First "createWindow" virtual protected method is getting called which does
not contain any URL of page but it will create WebEnginePage for the
WebEngineView.
Once the WebEnginePage is created in above method, we are getting
"acceptNavigationRequested" call which contains URL so before we save the
URL, tab was already opened so we are not able to compare with existing
opened tab.

I have tested this patch on Linux & Mac and it is working as expected.

Do review it and let us know for any comments.

Thanks,
Neel Patel