Animation UI: Make Ctrl+F use textbox instead of pop-up

Avoid blocking the UI when searching for animation channels with Ctrl+F.

Instead of showing a single text input in a blocking popup, Ctrl+F now
just focuses the search box above the channel list. It feels nicer to
use and has the niceties that come from using that textbox, like
searching per keystroke, compared to the old pop-up method.

As the behaviour of the operator has changed considerably, this also
changes the operator name from `anim.channels_find` to
`anim.channels_select_filter` and updates the keymaps.

Reviewed By: ChrisLend, sybren

Differential Revision: https://developer.blender.org/D12146