Update tmux.conf so it conforms to tmux3 & 2 style

[ldhieu]

The new format for tmux3 has listed two incompatible changes as follow (source)

INCOMPATIBLE: tmux's configuration parsing has changed to use yacc(1). There is one incompatible change: a \ on its own must be escaped or quoted as either \ or '' (the latter works on older tmux versions). INCOMPATIBLE: Add a new {} syntax to the configuration file. This is a string similar to single quotes but also includes newlines and allows commands that take other commands as string arguments to be expressed more clearly and without additional escaping.

So now {, } and \ have to be escaped to '{', '}' and '\' (to maintain compatibility with tmux2)

[mohitt]

The color background of the tab pane title has some issue, after making these changes on tmxu 3.0a version