live-preview: sidebar management

[szecket]

quite a few things have happened here:

I changed the sidebars so they float over the content and don't move it.
I animated this transition.
There is now a drop shadow (which has draw issues) under the sidebars
the content scroll area adjusts to exist where there is still visible area and allows for scrolling of all the content

[hunger]

This will conflict hard with your previous PR... let me try to sort that out.

[hunger]

I took the liberty to polish your PR a bit and pushed it into another branch for review:

https://github.com/hunger/slint/commits/szecket-flaps/

It sits on top of your earlier polish branch now and has the conflicts resolved.

I extracted several stand-alone parts (polish to the status bar, etc.), I would suggest to merge all of those in separate PRs (I can do that if you want). Then we only have the floating status bars to consider...

For those I got rid of the SidebarApi and streamlined things a bit. I still have some problems with the left sidebar: showing/hiding it sometimes moves the scrollview to far to the left, so that half the visible preview-area is dead space:-/

[NigelBreslaw]

If you are in preview mode and quickly resize the live-preview window you can see the properties pane show up. Make it visible false also in preview mode?