Alt-tab-macos: macOS-style rounded corners

[net]

Rounded corners would help the switcher blend into the OS.

[pcr910303]

Yeah, rounded corners and less prominent (grey instead of white) text colors will definitely make this more mac-esque instead of looking Window-ey.

[lwouis]

I actually prefer Windows style, but I understand preferences may vary. I think this should be a preference supported in some UI. There is already a ticket about that (https://github.com/lwouis/alt-tab-macos/issues/8), but I'll keep this separate one open to not have a god-ticket with a million features.

[lwouis]

Release 1.6.0 switches to a macOS theme. I kept the previous Windows 10 theme as a preference.

Hope you like it!

Note that I made the background a bit darker than the native app switcher because the text is difficult to read otherwise.