use getpref/setpref functions in user_string.m

Use [getpref][1]/[setpref][2]/[addpref][3]/[ispref][4] family of functions instead of manually maintaining preferences in text files.

Internally those functions persist values to a MAT-file at `fullfile(prefdir,'matlabprefs.mat'))`.

[1]: http://www.mathworks.com/help/matlab/ref/getpref.html
[2]: http://www.mathworks.com/help/matlab/ref/setpref.html
[3]: http://www.mathworks.com/help/matlab/ref/addpref.html
[4]: http://www.mathworks.com/help/matlab/ref/ispref.html