This patch adds a new FILE_OT_start_filter operator and a new ctrl+f shortcut.
It's fairly straight forward and based on TEXT_OT_start_find. The only difficulty
was to figure out, that I have to overwrite the context region temporarily to make it work.
Without this, the file browser would not receive any events anymore and appear frozen.
WM_event_add_ui_handler is eventually called by UI_textbutton_activate_rna and
expects C to reference the region that the activated uiBut is in.
Adding the User Interface team as reviewer, because this adds a new shortcut.