Consider removing legacy LangBar support in Windows #1595

Description

So-called "Language Bar" feature in Windows looks to be deprecated in practice:

• It has been disabled by default since Windows 8.
• It does not support dark-theme yet.
• New Microsoft IME for Japanese does not support it.
• Both New Microsoft IME for Japanese and ATOK have introduced their own LangBar-like feature instead.

Keep in mind that "Language Bar" is different from IME status icons on the task bar. You need to explicitly check "Use the desktop language bar when it's available".

With above, I think it would be acceptable for us to remove legacy "Language Bar" support from Mozc. Main benefits would be:

• Removal of legacy code, which is still consuming CPU cycles.
• Following icon files can be removed
  • src/data/images/win/tools_dictionary.ico
  • src/data/images/win/tools_dictionary_a.ico
  • src/data/images/win/tools_icon.ico
  • src/data/images/win/tools_icon_a.ico
  • src/data/images/win/tools_properties.ico
  • src/data/images/win/tools_properties_a.ico

Screenshots

Steps to reproduce

1. Enable "Use the desktop language bar when it's available" in the language settings.
2. Focus into some app with an input field.

Expected behavior

Language bar no longer shows Mozc's mode/tool icons.

Actual behavior

Language bar still shows Mozc's mode/tool icons as we used to do so in Windows Vista/7.

Version or commit-id

921b8cc

Environment

• OS: Windows 11 25H2