Don't expand widgets on hover (#7808)

* Part of https://github.com/emilk/egui/issues/5605

This changes the default style of egui.

The problem with "expanding" widgets is that they now want to paint
outside their own bounds, which then requires all parent UIs to have
proper margins.

It also means hovered things are no longer properly aligned with every
other widget.

crates/egui_kittest/tests/snapshots/menu/closed_hovered.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:27e4950f17ab7f68f7e001ca3a4f3fc18943103f4745c87715dcf6c097e92a57
-size 11131
+oid sha256:5b6447dd4bd6489b9b909b8240abe54db16331beaa2fb4656f01f79a08fb25f9
+size 11112

crates/egui_kittest/tests/snapshots/menu/opened.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:449cc473469dc80af81fe20e394dd90e67b4ae9c2033ebb7029726274d77d50c
-size 21644
+oid sha256:4baf110e63fb104f30e9ae06e3601cfb48d7b69aec31636ca470c9ba9f6d44a9
+size 21659

crates/egui_kittest/tests/snapshots/menu/submenu.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:1cbfc767ed169cbddb3c90c2b455daefd85925501e7e33c7a25a34a72fc13eac
-size 28512
+oid sha256:93d2f336e216f371e7239b08113e659bad6c30eb299a2d8ea9537ae1c63533f0
+size 28503

crates/egui_kittest/tests/snapshots/menu/subsubmenu.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:f4397b3d86bb5fe76d661cdb109ef71d43e771093bd9267b74722660d312ec7d
-size 33253
+oid sha256:6bf27cde6a87112a1ae832a65c80a9e34243b6ee368314379f5bf018edf439e5
+size 33239

crates/egui_kittest/tests/snapshots/readme_example.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:fad8b83e553ffa6bfbc4d47b955f2180859048c3789dda99b640e27665d216c7
-size 2244
+oid sha256:e86ed66f3ac3a81998eefbed8cb231edc6522174050676b45e9248f9e7f18533
+size 2227