Added sorting by name, date or size ascending/descending

Hello,
I have added the possibility to sort files/folders by name, date or size.
The option is stored temporary, so it is obeyed if the directory is changed.

Filename sorting fixes also #379 
Sorting by size: Folder are sorted by name, as there is no information about the number of files containing the directory.

Sorting is, like dolphin or windows explorer, separated between folder and files.

TODO: Translate to other languages.
Please, keep the convention of naming member variables with an 'm' prefix.

sortOrder -> mSortOrder
sortAscending -> mSortAscending
appPreferences -> mAppPreferences