Main page redesign and code cleaning.

Hello,
I have noticed you began your work on redesigning the main page, so I had a look at the code and created some improvements:
* ActionBar at the bottom for better reach (now it looks more like the Sailfish browser which users are used to)
* smooth ActionBar hiding animation (the current implementation of 'Behavior on visible' was invalid, as animation on boolean is not supported)
* corrected SilicaGridView to be on the main page, not inside SilicaFlickable. This is not only much cleaner and according to Sailfish code examples, but also resolved the problem of 'always visible PullDownMenu'.
* correct use of BackgroundItem as Grid delegate, instead of custom Image/MouseArea component that did not allow proper Sailfish press effect (as seen here https://sailfishos.org/sailfish-silica/sailfish-application-pitfalls.html )
* AttachedPage to replace the former overlay and to allow fast access to the profile with respect to Sailfish UI (AttachedPage is usually used for this purpose).

I hope you will like some of the changes. It is, of course, up to you to decide whether to include them or not. I can take a look at what can be improved at other pages (besides MainPage) if you do.