Blender Git Loki

UI: Make menu item use theme roundness

Menu items ignore the roundness setting since they spread left to right.
This patch makes it so menu items use the theme preference instead of
hardcoded square corners. Providing more flexibility to themes.

All built-in and included themes already have this set so no need to update them. For the default themes (Dark/Light) roundness is 0.4.

{F10950727, size=full}

The motivations behind this change are:
* To be more consistent with other widgets.
* Improve themes flexibility.
* Match padding with other elements that have like the Search field:

{F10950746, size=full}

Reviewed By: #user_interface, Severin

Differential Revision: https://developer.blender.org/D12813