change the minium font size setting globally

In my opinion, we can call `webPageStyleUpdated()` whenever the settings are saved, and require `Mainwindow::applyWebPageStyle()` to update them at one time. So, not only the `fontSize` settings, but also other possible settings in the future can be put into it.

The `minFontSizeChanged`/`changeMinFontSize` is useful, too. With them, users can see the change of font size immediately, without clicking the button.