If search string cannot be converted to ASCII (result is an empty string) use the original string (#1489911)
Fix message un-selection on the list (#1489906)
Fix debug output
Add option to suppress session data writes to DB
Merge branch 'master' of github.com:roundcube/roundcubemail
Improve performance of sort_folder_list() method.

Now sorting 25k folders takes around 3 seconds.
Bring back forcing of string type for folders list items (removed unintentionally by last commit)
CS fixes
Silence possible PHP Warning: file_exists(): open_basedir restriction in effect.
Silence PHP Warning: strtolower() expects parameter 1 to be string
Skip setting button state (css classes, etc.) if the state didn't change
- this fixes disappearing listmode selection after clicking search reset button