Consider using data URI scheme instead of credits_en.html

[yukawa]

Description

Spun off from #777.

Currently about_dialog.cc tries to open credits_en.html from SystemUtil::GetDocumentDirectory() as follows.

mozc/src/gui/about_dialog/about_dialog.cc

Lines 77 to 79 in b6aa2a3

	const std::string credit_filepath =
	FileUtil::JoinPath(SystemUtil::GetDocumentDirectory(), "credits_en.html");
	Replace(replaced, "credits_en.html", credit_filepath.c_str());

However it means that each installer (including the one made by Linux distribution package maintainers) needs to correctly install credits_en.html into the target system, which is fragile and hard to maintain as we've seen in #777. Let's see if we can use data URI scheme to embed the data into mozc_tool executable or not.

Steps to reproduce

1. Open mozc_tool
2. Click 'open source software'

Expected behavior

See the opensource license information without relying on credits_en.html.

Version or commit-id

b6aa2a3

Environment

• OS: all desktop

[yukawa]

It turns out that this is not possible at least in Windows due to the size limit of CreateProcessW.

The maximum length of this string is 32,767 characters, including the Unicode terminating null character.

Unfortunately our credits_en.html is already too big to be passed with CreateProcessW. So closing as "not planned" for now.

For future reference, we could still embed credits_en.html into mozc_tool by showing it with either QTextDocument::setHtml() (or converting it to markdown then showing it with QTextDocument::setMarkdown()).