[GUI] Limit amount to 8 decimal digits

[pythcoiner]

This PR replace #977 and fixes #798
I've followed jp1ac4 advice and create a Form::new_amount_btc() method that act like Form::new_trimmed() w/ few more (filtering) features:
- allow only input of "0123456789,." characters
- "," is replaced by "."
- only one "." separator
- maximum 8 digit after separator

these features works for keyboard input or paste

i've tryied to add a "0" in case the String start with a "." but this make the cursor have a bad location and i do not find a way to control the cursor location w/ iced, so i revert this feature

[pythcoiner]

not related to this PR: should we add gui/Cargo.lock to .gitignore?

[jp1ac4]

Instead of using regex, another option might be to use bitcoin::Amount::from_str_in to determine if the new string can be parsed as a BTC amount and then use this to decide whether to pass this updated value to the on_change function or keep the previous value. I see we already have bitcoin as a dependency in the ui project so wouldn't need to add anything.

[darosior]

I agree with @jp1ac4, let's not reinvent the wheel down here if they already have parsing upstream.

[pythcoiner]

i've done adviced changes, not yet tested because my crash issue

[jp1ac4]

Changes are working well. Just had a couple of comments.

[pythcoiner]

adressed comments & squash

[edouardparis]

Please remove the unused regex dep. Thanks

[pythcoiner]

Please remove the unused regex dep. Thanks

oops, done & rebased on master

[jp1ac4]

ACK 9415dc9.

[edouardparis]

ACK 9415dc9