Per-widget style (#7667)

* Closes <https://github.com/emilk/egui/issues/7586>

Implementation of the per-widget style of the action plan to add better
styling option to egui

---------

Co-authored-by: adrien <221212@umons.ac.be>
Co-authored-by: Adrien Zianne <adrien@iq002.ipa.iqrypto.com>
Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

tests/egui_tests/tests/snapshots/visuals/drag_value.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:5ca946ae1875730db15a7e525d2edfab4b55d9a07ad72998c565ce0c7c9bea90
-size 8400
+oid sha256:540388365970accbc4c03aa34809b1f07d4e31c8d56bccb8f73da9c2292cb36a
+size 8426

tests/egui_tests/tests/test_widgets.rs

@@ -339,7 +339,7 @@ impl<'a> VisualTests<'a> {
 
         f(&mut harness);
 
-        harness.step();
+        harness.run();
 
         let image = harness.render().expect("Failed to render harness");