Style tweaks (#2406)

* Note the namechange of egui::color to egui::ecolor

* Use a solid triangle for collapsing headers and windows

* Add Shadow::NONE

* Add Visuals::panel_fill, window_fill and window_stroke

* Bug fix: ComboBox::width sets the outer width of the ComboBox

* egui_extras::Table: add functions to access the `Ui` for the header/body

* ComboBox: use solid triangle

* Tweak default menu margin

* Nudge panel separator lines so they stay visible

* Update changelogs

crates/epaint/src/shadow.rs

@@ -14,6 +14,11 @@ pub struct Shadow {
 }
 
 impl Shadow {
+    pub const NONE: Self = Self {
+        extrusion: 0.0,
+        color: Color32::TRANSPARENT,
+    };
+
     /// Tooltips, menus, …, for dark mode.
     pub fn small_dark() -> Self {
         Self {