[pgAdmin4] - Desktop runtime patch

Hi,

Please find the attached patch file for the below fix when pgAdmin 4 run in
desktop mode.

Added functionality to open different website link and load the website
data to different tab. To achieve this, new customized QTabWidget and
QWebView are added to render the data to WebView widget. All the widgets
(New Tab, WebViewWinodw, and QToolButton) are added dynamically.
QToolButton is used to traverse back and forward to web document opened in
WebViewWindow.

Introduced the New class called WebViewWindow which is derived from
QWebView. Each tab of the QTabWidget contains the instance of WebViewWindow
class. WebViewWindow class is useful to display the web document.

Introduced New class called TabWindow which is derived from QTabWidget. This
class is useful to achieve following functionality.
- Customize the close button of tabbar so that it can only be visible other
then main pgAdmin 4 window.
- Enable/Disable the toolbutton added left side of tabbar depending on
the web history traversed by the user in WebViewWindow.
- Set the tooltip text of the tabbar depending on the title change event
of WebViewWindow class.

Modified the Qt project file to support the both the version of python 2
and python 3.

Qt5 is recommended to test pgAdmin4 in dektop mode.

Attached patch is applied and tested on Linux, do let us know for any
comments/issues.

Thanks,
Neel Patel