Fix occasionally sending duplicate messages

[RxmTaco]

This patch fixes #868 by replacing enter key handling in message_input.dart to only happen during KeyDownEvent instead of the global isLogicalKeyPressed state.
In chat.dart the state for the processing flag in sendMessage was already being set, but was not used to guard the operation by checking it's state before processing a new message. This has been changed to return early if a previous message is still being processed while a new sendMessage gets requested.

I was not able to reproduce the error myself and thus wasn't able to verify that the patch works as intended.
However @Katacc originally let me know of the problem and also was able to test and verify that the patch works.

[Katacc]

I can confirm testing this with @RxmTaco patch.

Previously messages would be sent twice seemingly randomly, but it had to do with sending too quickly after typing. With the patch this couldn't be reproduced anymore.

+1 from me

[Airyzz]

i believe this was already fixed in #838, which would explain why you couldn't reproduce it. this change might not be necessary

[Katacc]

i believe this was already fixed in #838, which would explain why you couldn't reproduce it. this change might not be necessary

This is probably not yet in the latest flatpak?

[Airyzz]

This is probably not yet in the latest flatpak?

that's correct

[RxmTaco]

i believe this was already fixed in #838, which would explain why you couldn't reproduce it. this change might not be necessary

Oops, didn't see that one! Still I would think using the KeyDownEvent would be wiser than relying on a debounce time, since the event only triggers once during a key press mitigating the need for debounce. Guarding the sendMessage with an early exit using the processing flag shouldn't do any harm either, but your choice if you want to add these.