plasmoid: Improve the size and margins of buttons inside of Screen Layout configuration

Reason for the change

The buttons keep shifting, when you resize the window.

This looks really unprofessional.

The leftover from Math.floor throws away the remainder at the end.

After the change the leftover goes into the button widths (±1px) instead into the right margin (±4px).

I also made sure that the buttons are centered by adding right margin (there was only left margin before, now it has both left and right margin)

Test plan

1. Resize the Display Configuration applet while watching at the right side of the window to observe the buttons shifting
2. Load the fix and observe that the buttons don't jump now

Screenshots or screen recordings

Before the change

window width	left gap	right gap	button widths	inner gaps
390	4px	0px	74, 74, 74, 74, 74	4, 4, 4, 4
391	4px	1px	74, 74, 74, 74, 74	4, 4, 4, 4
392	4px	2px	74, 74, 74, 74, 74	4, 4, 4, 4
393	4px	3px	74, 74, 74, 74, 74	4, 4, 4, 4
394	4px	4px	74, 74, 74, 74, 74	4, 4, 4, 4
450	4px	0px	86, 86, 86, 86, 86	4, 4, 4, 4
523	4px	3px	100, 100, 100, 100, 100	4, 4, 4, 4
601	4px	1px	116, 116, 116, 116, 116	4, 4, 4, 4

After the change

window width	left gap	right gap	button widths	inner gaps
390	4px	4px	73, 73, 74, 73, 73	4, 4, 4, 4
391	4px	4px	73, 74, 73, 74, 73	4, 4, 4, 4
392	4px	4px	74, 73, 74, 73, 74	4, 4, 4, 4
393	4px	4px	74, 74, 73, 74, 74	4, 4, 4, 4
394	4px	4px	74, 74, 74, 74, 74	4, 4, 4, 4
450	4px	4px	85, 85, 86, 85, 85	4, 4, 4, 4
523	4px	4px	100, 100, 99, 100, 100	4, 4, 4, 4
601	4px	4px	115, 116, 115, 116, 115	4, 4, 4, 4