Avoid deadlocks by using lambdas for context lock

[emilk]

It used to be quite simple to cause dead-locks using egui. For instance:

if let Some(point_pos) = ctx.input().pointer.latest_pos() {
     // context lock is still being held here, so any call to the egui context would deadlock
}

With this PR you must now write this as:

if let Some(point_pos) = ctx.input(|i| i.pointer.latest_pos()) {
     // context lock is NOT held here, so you wont dead-lock
}

This is a bit more noisy, but far less error prone. The Context lock is held for the duration of the lambda, so keep them short and don't call other context-related things inside them.

Other examples:

• ctx.input().key_pressed(Key::A) -> ctx.input(|i| i.key_pressed(Key::A))
• ui.memory().toggle_popup(popup_id) -> ui.memory_mut(|mem| mem.toggle_popup(popup_id))

Most of the work in this PR comes from @MaximOsipenko (in #1969).

Closes #2064
Closes #2546 (maybe)

TODO

• Check for any lingering mentions of the old style in markdown files or comments
• Update CHANGELOG.md
• Write docs for the functions
• Write a migration guide
• Remove ShortRwLockHelper
• Write a better PR description, including rational, alternatives, and migration guide
• Self-review the PR
• Try it out in Rerun