Fix PopupPanel and PopupMenu menu styles

[Giganzo]

Fixes: https://github.com/godotengine/godot/issues/96517 Edit: Fixes: https://github.com/godotengine/godot/issues/67657 (border part)

This makes popup menus using PopupPanel and PopupMenus have same base look, as it is in Godot 3.

Panel border

Adding a default border to the PopupPanel to make it easier to differentiate from the background. I believe this was removed in PR https://github.com/godotengine/godot/pull/45607 and then added back for PopupMenus in PR https://github.com/godotengine/godot/pull/48655 (also removed support for Border Size on PopupMenus)

Before:

After:

Border color

In Godot 3 both styles of popup menus uses the same border color, light on dark theme and dark border on light themes.

Godot 3 example:

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Currently Godot 4 uses dark border for PopupMenus and with Border Size in Editor Settings set to 1 or 2 PopupPanel uses a light border in dark themes.

Before: with border size set

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After: border size is set to deafult

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Because the PopupMenu border is visible by default I went with that color for both in this PR.

Maybe there could be a editor setting for changing outline color, but that would need to be a proposal in that case.

Border Size

In Godot 3 both PopupPanel and PopupMenus uses the Border Size.

PopupPanel uses this already.

This PR also changes PopupMenus to use the editor setting Border Size for the outer border.

Separator width

Due to the change of using Border Size the separator line in PopupMenus was much more obvious that it overlaps the outer border. So this was also changed

Before:

After:

Compact

Before:

After: