make windows feel more native

[szymonkaliski]

Why

• covering the screen fully without gaps feels off
• cmd-shift-n instead of cmd-n for new window is weird too
• new windows spawning exactly over existing ones looks weird as well

What changed

• switched cmd-shift-n to cmd-n
• made it so new window without any past stored config has some default size, and if too big to fit the display, it shrinks
• finally new windows spawn with some offset

Test plan

As described above.