Push dialogs as pages and redesign tabs selection

Sorry for larger MR, I hope that's OK. But let me know if its too much.

This originally started as a simpler rewrite for pushing Tabs, Settings, and other secondary pages into page stack as pages instead of layers. This approach allows to swipe the unwanted dialog off the screen, making it easier to use on mobile. As there is an animation shown when pages are pushed, its intuitive as well. The functionality is in the first commit.

Note that the web page is using Kirigami's fillWidth functionality as well as pinning the first page.

Next, I worked on restoring live previews of tabs overview and redesign of tabs selection. UI is following Jolla's selector which works very well. In addition, new tab action is moved into the main action of the page, making it easy to add it when needed.

The selector is currently showing stripes with page regions, one after another in a single column. In landscape, that's not good enough. In portrait, works very well.

• Support landscape mode. For that, I am planning to allow 2 or 3 tabs per row in the grid in landscape mode.
• Avoid having all WebViews working all the time, prefer to have visible:false for non-active ones.

The second item is an optimization issue. I may know why grabToImage failed for me earlier, but will have to check it. Alternative is to use non-live ShaderEffectSource associated with WebView.