A breakpoint stop now grabs attention like IDEA's: the owning window's
taskbar button flashes while the window is in the background (new
window_flash_taskbar command wrapping request_user_attention, which never
flashes a focused window), and the flash is stopped when the window is
focused, the paused thread resumes, or the session ends. A window running
with Focus application on breakpoint still focuses instead of flashing,
and repeated hits while already flashing do not re-request attention.
Closes#1742
Signed-off-by: Inf1mum <[email protected]>