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.
Integrated new splash screen

Also removed ugly code for displaying translucent splash images.
AutomatableModel: added centerValue property

The optional centerValue property can be used by views to display the
center somehow. It defaults to minimum value.
Knob: draw arc around knob

Initial work on improved knob usability by drawing an arc around knob
indicating current state as well.
InstrumentTrack: set center values for pitch and panning
Knob: draw arc + arc background for all unstyled knobs
EnvelopeAndLfoParameters: added center values for amount models
Knob dots by Tobiasz
Revert knob dots, make knob arc use palette, remove bb-editor image... bye bye TR-808 :(
Merge pull request #122 from diizy/stable-0.4

Some UI stuff
Loop marker: Changed tooltip text.
Update timeline.cpp
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
Make LFO controller accept custom waveforms
Tooltip: Changed tooltip text of LFOControllDialog of custom waveform
Fixes Ghost Peak Controller Bug https://github.com/LMMS/lmms/issues/115
About window: Make homepage link clickable