Dont include ALL txt files from root directory

[rahulporuri]

This PR changes the manifest file to only include the image license and other relevant top level files, not all txt extension files.

On master, the following additional files are included in the sdist, which aren't included in this PR.

• appveyor-clean-cache.txt
• CHANGES.txt
• ci-src-requirements.txt
• dev_requirements.txt
• doc-src-requirements.txt

[codecov-io]

Codecov Report

Merging #816 (1cecfe4) into master (0d1516e) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #816      +/-   ##
==========================================
+ Coverage   40.81%   40.89%   +0.07%     
==========================================
  Files         508      508              
  Lines       27845    27847       +2     
  Branches     4219     4219              
==========================================
+ Hits        11366    11388      +22     
+ Misses      15999    15986      -13     
+ Partials      480      473       -7     

Impacted Files	Coverage Δ
pyface/ui/wx/tree/tree.py	0.00% <ø> (ø)
pyface/i_widget.py	100.00% <100.00%> (ø)
pyface/ui/qt4/console/console_widget.py	29.28% <0.00%> (+0.20%)	⬆️
pyface/ui/qt4/code_editor/code_widget.py	43.97% <0.00%> (+0.42%)	⬆️
pyface/wx/python_stc.py	10.36% <0.00%> (+1.21%)	⬆️
pyface/ui/wx/window.py	77.10% <0.00%> (+2.40%)	⬆️
pyface/ui/qt4/util/gui_test_assistant.py	80.53% <0.00%> (+2.65%)	⬆️
pyface/ui/wx/data_view/data_view_widget.py	77.52% <0.00%> (+5.61%)	⬆️
pyface/__init__.py	88.88% <0.00%> (+7.40%)	⬆️
pyface/i_file_dialog.py	84.61% <0.00%> (+7.69%)	⬆️

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[mdickinson]

LGTM