Improve html/sidebar UX

[pogoba]

• show sample screenshot to hint people on what they are supposed to screenshot
• add limitations: Only those iconpacks, only those languages
• await all button presses properly and disable buttons as often as possible
• alert on parsing error messages
• more detailed progressbar
• add CSS

[GICodeWarrior]

Some of this was included in #31. Closing this as completed, but feel free to reopen or open a new issue if anyone wants more. :-)