Added two standard paths to file open/save dialog

In particular Desktop and Downloads have been added.
Also introduced a new function to create a file dialog
in order to prevent code duplication.
MainWindow: Moved FileDialog to own subclass
Update code to use FileInfo instead of QFileInfo

Also removed all blocks similar to
```
#if QT_VERSION >= 0x040806
	filedialog.setOption( QFileDialog::DontUseCustomDirectoryIcons );
#endif
```
as this now takes place in the new subclass's constructor.
Add new class VersionedSaveDialog

A file save dialog (inherits FileDialog) that provides buttons to
increment or decrement a version which is appended to the file name.
(e.g. "MyProject-01.mmpz")
Add "Save as new version" action to project menu

Use Shortcut Ctrl+Alt+S. Part of issue #86
Merge pull request #98 from Lukas-W/stable-0.4-iss90

Update file open/save dialogs