[FIX] window-size: fix error on loading screen

[vcapretz]

the window size (height/widhth, x/y) should all be integers

if window tried to load with float numbers it was throwing errors
on the main process.

@RocketChat/DesktopApp

Closes #540

[alexbrazier]

Might be better to do the the Math.floor in the save state function instead? Also, if we always save the state with Math.floor then it shouldn't be necessary to do it again when retrieving the values. But approving anyway as it is ok as it is.