Explicitly specify `signed` modifier for `char` type

Version 4.0.1.

- Fix window title on Windows 7.
- Bug fixes and other minor improvements.

Version 4.0.

- Premium: Send media and files each up to 4 GB in size.
- Premium: Download media and files at the fastest possible speed,
with no limits.
- Premium: Double most of the limits in the app.
- Premium: Get a new button next to any voice message
to generate a transcript of its audio.
- Premium: React with even more emoji,
including :clown: and 😍.
- Premium: Send unique stickers with additional effects,
updated monthly.
- Premium: Set a default chat folder
or enable tools to auto-archive and hide new chats.
- Premium: Subscribers have a badge next to their name,
showing they help support Telegram.
- Premium: Show off your profile video
that will be animated for everyone in chats and the chat list.
- Premium: Sponsored Messages that are sometimes shown
in public channels will no longer appear.
- Enable join requests for your public groups,
no invite links required.
- Users who open the group will see an Apply to Join Group button.
- Once approved by an admin,
users will be able to participate in the chat.
- Bots that are integrated into the attachment menu
can be programmed to work in groups and channels.

Beta version 3.7.6: Backport a Qt 5.15.4 crashfix.

Beta version 3.7.5: Fix MSVC x64 Release build.

Beta version 3.7.4: Update tg_owt in snap.

Version 3.7.3: Don't copy text from a restricted post.

Version 3.7.2.

- Fix mute period selector values.
- Fix a crash in repeated context menu item selection.
- Fix context menu item selection of systems without a compositor.

Version 3.7.1.

- Hardware accelerated video decoding off by default.
- Fix several crashes.

Version 3.7: Fix a crash in some langpacks.