Launching screen

A "Welcome to OpenSCAD" dialog box is added as a launcher in openscad. It will show up as the openscad launches, giving options to the user as his first action.

ToDo:
* [x] Change window title (maybe "Welcome -- OpenSCAD"?)
* [x] Handle dialog resizing -> currently set to fixed size
* [x] Don't open dialog if a filename was given on command line when starting OpenSCAD
* [x] Check if there's a way to force the file dialog ("Open" button) in front of the dialog (happens with Debian/Gnome, not on Ubuntu)
* [x] Don't Open the main window until after the launcher has finished
* [x] fix compilation with ENABLE_MDI is set (this is default on Mac)
* [x] On Mac, the launcher somehow doesn’t get input events unless I first click into the main window, and then change focus back to the launcher
* [x] Check small visual artifact in the Open buttons (see MacOS screenshot)
* [x] The Recents list shows files with full paths, so it’s hard to read, find a way to improve
* [x] Change "Show this everytime" to "Don't show again"
* [x] Move Launcher preference from Features tab (to Advanced?)
* [x] Make the "Open Recent" button have a visual difference when disabled
* [x] Fix tooltip issue (see screenshot in comments)
* [x] Enable Cmd-Q / Ctrl-Q ? -> leave it as it is for now, looks like a Qt issue is interfering
* [x] Tune the title style to look more like the website
* [x] If I deselect a recent or example entry, the corresponding "Open" button isn't greyed out -> disable deselect or implement greying out on deselect -> this is a Qt issue and only happens on Mac

Linux Screenshot (Debian / Gnome)
![openscad-launch-screen](https://cloud.githubusercontent.com/assets/1330241/3788067/e8748b90-1a4f-11e4-89f2-772922bf2aa9.png)

MacOS Screenshot
![screen shot 2014-08-09 at 22 49 22 pm](https://cloud.githubusercontent.com/assets/1330241/3869764/9d10a1c8-20a5-11e4-915a-f231bdacec0a.png)