applets/webbrowser: don't show popup block message on every page

The applet listens to onNavigationRequested, and if emitted, shows the popup warning. navigationRequested, however, is emitted on every page load, including initial load and normal clicking of links, so the button is always visible and will claim that it blocked a kde.org popup even if the site does not have any. (The only reason normal page loads aren't opened in an external browser is that WebEngineNavigationRequests don't actually have a userInitiated property).

Instead, we listen to onNewWindowRequested - this is only emitted if there is actually a new window involved, and has the userInitiated property we're using. This way, the "popup blocked" button only appears if a popup was actually blocked. We also remove the button's attached Tooltip - the button has a ToolTip object with extra information, and as it we get two tooltips.

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