Add button in GUI to export a single channel by ID

[AlmightyLks]

Reason for PR

Users tend to get frustrated or impatient with the GUI which loads all guilds and all channels. Especially when including Threads.
The CLI, which can easily do this selectively, however is way too complicated for laymen who have never seen an open command line.
Thus, for accessibility reasons, we could allow the quick-export functionality of a given specific channel id via the GUI as well.

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Changes:

New starting view, added the flash button:

The Flash Button toggles a new input, which allows to export a single channel via a given Id:

Which functions like hitting Export on a singular channel when going for the elaborate way through the list of all guilds and all channels:

[Tyrrrz]

Hi. First of all, thanks for the PR. It looks really nice.

I have some suggestions regarding the UI:

1. The lightning icon, I fear, is not super descriptive. How about using the pound symbol icon (#)? Could use the outline variant so as to not get confused with the channel icon in the channel list. I think the # symbol is great for this functionality because it's associated in Discord with IDs.
2. Instead of expanding the top section to fit another textbox, let's just show a new dialog instead.
3. On that dialog, we should allow the user to enter multiple channel IDs by separating them with newlines. Somewhat similar to this: https://github.com/Tyrrrz/YoutubeDownloader/blob/ba1bf900263b5ca64702ef34f1e375b44c0cdf0b/YoutubeDownloader/Views/Components/DashboardView.axaml#L22-L30

[AlmightyLks]

Hey

1. I suppose, easy fix
2. That's what I feared, as this initial way via the main window was the easiest and fastest, where all dependencies were already in place 😆This might take me a second, because I am not used to Desktop Dev, XAML nor Avalonia (or WPF)
3. Aight

Still WIP

[Tyrrrz]

Hey there @AlmightyLks.

Are you planning on continuing with this PR or do you no longer have time/interest for it?

[AlmightyLks]

Speaking for myself, I honestly will drop the PR. Maybe someone else picks up on it or something. 😄

[Tyrrrz]

Okay, no worries. Thanks for putting in effort :)