TeXmaker as default application to open tex files
Solution 1:
If you can not associate from GUI - you may try to set tex-file association in configuration file ~/.config/mimeapps.list .
Add (or change existing) line
text/x-tex=texmaker.desktop;
to the section
[Added Associations]
Solution 2:
Open properties for any .txt file. Select "Open With", "Texmaker". Select "Make Default". This will make Texmaker the default for all text files.