Add an option to disable Autocomplete for text files

[dzeng21]

#5296

This pull request adds an option to disable Autocomplete for text files only.

Changes:

• Added a checkbox, under the autocomplete settings, to disable autocomplete for text files
• Check if a file is a .txt file upon switching files or file rename

Notepad++ will now detect if a file that is opened or in focus is a .txt file (or renamed with .txt) and disable or enable autocomplete accordingly, depending on if the option is checked.

[vinsworldcom]

@dzeng21 , please add the word "Fix" before your #5296, this will auto-link the PR to the issue and make it easier for @donho see and likely (hopefully) approve.

[donho]

#5296 (comment)