Fix reset to Poll after the event loop starts

[daxpedda]

#3056 introduced the possibility of setting ControlFlow before the event loop starts. This was not taken into account before #3056, therefore some code existed that assumed that it always starts with ControlFlow::Poll.

This PR corrects or removes these assumption and fixes the bug that ControlFlow is always reset to Poll after the event loop starts. As a side effect, ControlFlow is also not reset to Poll after pump events & co.

I also moved some of the set_control_flow() calls in examples and documentation up to EventLoop instead of having to call them inside run() on EventLoopWindowTarget.

Discovered in #3106.