[lldb][gui] truncate long lines/names if needed

Without this, sources with long lines or variable names may overwrite
panel frames, or even overrun to the following line. There's currently
no way to scroll left/right in the views, so that should be added
to handle these cases.
This commit includes fixing constness of some Window functions,
and also makes PutCStringTruncated() consistent with the added
printf-like variant to take the padding as the first argument (can't
add it after the format to the printf-like function).

Differential Revision: https://reviews.llvm.org/D85123