Add Developer Tools to mouse context menu

[cztomczak]

Set CefSettings.remote_debugging_port to a random value between 49152 and 65535 to enable developer tools.

The C++ code for showing devtools is:

bool ShowDevTools(CefRefPtr<CefBrowser> browser) {
    CefWindowInfo windowInfo;
    CefBrowserSettings settings;
    #if defined(OS_WIN)
    windowInfo.SetAsPopup(browser->GetHost()->GetWindowHandle(), "DevTools");
    #endif
    browser->GetHost()->ShowDevTools(windowInfo, browser->GetHost()->GetClient(), settings);
    return true;
}

The C++ code for adding a menu item is:

CefContextMenuHandler::OnBeforeContextMenu() {
    model->AddItem(_MENU_ID_DEVTOOLS, "Show Developer Tools");
}
CefContextMenuHandler::OnContextMenuCommand() {
    if (command_id == _MENU_ID_DEVTOOLS) {
        ShowDevTools(browser);
        return true;
    }
}

[cztomczak]

In the meantime to enable remote debugging one would need to add this code:

cefSettings.remote_debugging_port = 8080

Here after line 125:
https://github.com/CzarekTomczak/cef2go/blob/f78983456b6c8a899d27f72ffe6923019ba92504/src/cef/cef.go#L125

This will allow to open the cef2go developer tools in Google Chrome at the http://127.0.0.1:8080/ address.