refactor display when kindlegen is missing

[darodi]

refactor display when kindlegen is missing
display_kindlegen_missing

• fixing links
• same display called in 2 contexts

[darodi]

• fix Updates GUI text for new Homebrew version #491

[axu2]

EDIT: fixed kindlegen not being found when running from source in #560

I tried running this from source on Mac.

Just brew installing the cask of comic creator/kindle previewer is not enough. You also need to copy kindle gen to /usr/bin/local (the PATH equivalent on Windows), like the wiki says here:

https://github.com/ciromattia/kcc/wiki/Installation#kindlegen

Also updated the kindlegen sections and added a install from source section

I recommend we delete the OS specific code and just link directly to the wiki, similar to how we do for 7z. E.g. don't link to KindleGen or Homebrew or 7z, especially since there's no direct download of kindlegen anymore, that way we only need 1 line on the UI. Also wiki is easier to update.

Also, a common questions I've seen is

Where is the KCC directory?

Since now we just offer an exe download, so a wiki page could clarify.

@thatrobotdev

[axu2]

#463 has more info.

Looks like you can't even brew cask install Kindle Previewer? So maybe just link directly to Kindle Previewer.

2023-07-19 12:37:31.396 installer[50980:608557] Package /usr/local/Caskroom/kindle-previewer/3.72/KindlePreviewerInstaller.pkg uses a deprecated pre-10.2 format (or uses a newer format but is invalid).
installer: Package name is Kindle Previewer 3.72.0
installer: Installing at base path /
installer: The install failed. (The Installer could not install the software because there was no software found to install. The Installer could not install the software.)

Actually the Kindle Comic Creator webpage looks like it hasn't been updated in like 10 years. The webpage still references Windows Vista and OSX Snow Leopard. Should probably recommend it as a backup since it contains an older version of kindlegen:

Though I suppose this could all go in separate PR.