Inconsistent API for margins

**Is your feature request related to a problem? Please describe.**
As new user of `egui`, I am frequently met with major and minor inconsistencies in the crate API. One such inconsistency is the occasional usage of `Vec2` for margins rather than the dedicated `Margin` struct.

**Describe the solution you'd like**
I am planning to try and fix the aforementioned inconsistency with the margin API by switching as many of the (legacy?) `Vec2` implementations over to `Margin`.

**Describe alternatives you've considered**
I don't see any alternative to this other than not doing anything.

Start by searching the egui crate for APIs that use Vec2 for margins and compare them with the dedicated Margin struct. Identify which usages are legacy and determine the consistent set to update; done means the affected margin APIs use Margin without breaking existing behavior, with the relevant existing tests passing.

Written by the indexing model from the issue text.