Fit the slider value box to its text instead of a fixed 50px

[MikalaiKryvusha]

DrawRangeField draws the number box next to a slider, and the percent label, with a fixed GUILayout.Width(50). With a bigger font (a 4K screen, or a custom skin through WindowSkin) the value gets clipped (-0., 0.() and 100% wraps to a second line.

The width now fits the text in the current style, and one width is shared by all of them so they line up (per review):

• it is measured on the longest of the current value, the minimum and the maximum of the range, plus the style's margin (the percent label sits inside a horizontal group, and without the margin 100% still wrapped);
• it is measured on a sample of zeros of that length, not on the value itself, so the box does not jump while the slider is dragged;
• the width is global and only grows: a value that needs more room bumps it for every slider, up to 10 characters; it is reset when the window opens or closes, so a font or skin change is picked up;
• it is never narrower than the old 50, so the default look does not change.

Tested in a Unity 2020.3 game: at 3840×2160 with a large font 480.0, 2.0, 0.7 fit whole and 100%, 25% stay on one line; at 1280×720 all value boxes and percent labels stand in one column of the same width.

This is the last one from me, thank you again!