Make desktop clients look more native: use transparency

[camilasan]

TODO:

• add backdrop shadow
• make search bar more round, e.g. like this:
• make the dropdowns more rounded, look more native, e.g. like this:

Change it for:

• Mac
• Windows
• Linux

See #6639

[camilasan]

Fixed with #6980

[jancborchardt]

This is actually about transparency, while #6980 is about the buttons – we should reopen this, no @camilasan?

[Rello]

the corners were adjusted and the window is having a shadow by itself already by the macOS e.g.
is there something still open?