IDLE - Update mousewheel event handling

[terryjreedy]

Followup to gh-153741 which made IDLE aware of x11 wheel event change in 8.7 and 9.0. Move definition of x11_buttons from test_sidebar to util. Move def wheel_event from tree to util and move its test from test_tree to test_util. (I expect tree.py to eventially be replaced by ttk.treeview.) Make various other adjustments

There are unneeded bindings of buttons 4 and 5 in sidebar. We might be able to handle the needed binding with wheel_event with a second argument. I will leave this for a second PR.

Linked PRs

• gh-154511: IDLE - Update mousewheel event handling  #154512
• gh-154511: Consolidate IDLE's mouse wheel handling in util #156974
• [3.15] gh-154511: Consolidate IDLE's mouse wheel handling in util (GH-156974) #157021
• [3.14] gh-154511: Consolidate IDLE's mouse wheel handling in util (GH-156974) #157022
• [3.13] gh-154511: Consolidate IDLE's mouse wheel handling in util (GH-156974) #157023