Fix input event handling

[iscke]

this somewhat fixes a particular bug I've experienced in text handling, in which as far as i can tell:

• the user triggers dialogic_default_action on the same frame they also trigger another input action (say, moving the mouse)
• _input is called for the dialog_default_action, dialogic_default_action has been pressed this frame, so the coroutine to advance text (handle_event) starts
• _input is called for the other action, dialogic_default_action has been pressed this frame, so the coroutine to advance text starts again
• the coroutines resolve, text is advanced twice, and a line gets skipped

i believe this PR fixes the input situation, but the race condition still probably exists. i don't understand how godot godot coroutines work well enough to tell if it's actually an issue, though.

[Jowan-Spooner]

Thanks!