Fix DPropertySheet padding on bottom edge.

[AbigailBuccaneer]

Previously the bottom padding on the panel in a DPropertySheet was exactly 8px, because the active tab is 8px taller than the other tabs¹. However, the default padding is 8px, so this is only revealed when changing the padding on a property sheet.

[AshleighAdams]

It'd be better if you didn't use magic numbers.

[AbigailBuccaneer]

@Kateadams the 8px that's added by the active tab is a magic number. I went for the most minimal fix instead of also modifying the tab class to not use a magic number. I'll change it. (In fact, given the y-position is hardcoded to 20 + self:GetPadding() regardless of the height of the tabs, it probably makes more sense to use that as the top offset.)

[AbigailBuccaneer]

Whoops - I managed to automatically close this and make it unopenable again by updating my AbigailBuccaneer:patch-1 branch in line with garrynewman:master. Apologies, I'll open a new pull request now.