Add method display_message to Pane

[ritiek]

Useful for displaying messages in a tmux session and doing stuff like:

>>> pane.display_message('#T', get_text=True)
'server.py (~/Downloads/libtmux/libtmux) - NVIM'

Here is a complete example:

# First create a Tmux session by yourself
# before running this example
# `$ tmux`

>>> import libtmux
>>> server = libtmux.Server()
>>> session = server.list_sessions()[0]
>>> pane = session.attached_pane
# Display a message "Hallo World!" in the tmux session
>>> pane.display_message('Hallo World!')
# Get current terminal title
>>> pane.display_message('#T', get_text=True)
['server.py (~/Downloads/libtmux/libtmux) - NVIM']

[codecov-io]

Codecov Report

Merging #119 into master will decrease coverage by 0.25%.
The diff coverage is 25%.

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
- Coverage   79.81%   79.55%   -0.26%     
==========================================
  Files           8        8              
  Lines         857      861       +4     
==========================================
+ Hits          684      685       +1     
- Misses        173      176       +3

Impacted Files	Coverage Δ
libtmux/pane.py	82.81% <25%> (-3.86%)	⬇️

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