Scintilla Editor

This adds a new code editor based on QScintilla2.
More info:
* https://github.com/openscad/openscad/wiki/ScintillaEditor-TODO-list
* https://github.com/openscad/openscad/wiki/GSoC-2014-Project-Timeline

TODO before merge (less critical issues are collected in #915):
 - [x] Use the font and size set in Preferences
 - [x] CTRL+Mouse Scroll only works with the old editor when enabled in preferences (was fixed in e3960304bdc274f1c138dabb33a25abb1b4dfe3c)
 - [x] Get Find/Replace functionality working
 - [x] Fix ```Paste Viewport Translation``` menu entry
 - [x] Fix ```Paste Viewport Rotation``` menu entry
 - [x] Implement (or hide menuentries) for ```Indent```/```Unindent``` - currently hidden
 - [x] Implement (or hide menu entries) for ```Comment```/```Uncomment``` - currently hidden
 - [x] Fix shortcut issues (see below)
 - [x] Remove -std=c++11 warnings
 - [x] Changing scintilla editor font doesn't work on Windows -> false alarm - was a build mistake
 - [x] Fix Drag&Drop of files to the legacy editor
 - [x] Document change is not detected in the scintilla editor
 - [x] Build system for QScintilla2 on Mac
 - [x] Build system for QScintilla2 on Linux
 - [x] Build system for QScintilla2 on Windows
 - [x] Test snapshot build on Mac
 - [x] Test snapshot build on Linux
 - [x] Test snapshot build on Windows
 - [x] Code review (@kintel)
 - [x] Scintilla incorrectly reports source changed when opening a file
 - [x] Fix weird Windows issue (http://forum.openscad.org/Dev-Snapshots-with-the-new-editor-component-tp9486p9499.html) -> moved to #930

**Shortcut issues**

QScintilla defines a whole bunch of editor shortcuts (see https://github.com/H-uru/PlasmaShop/blob/master/QScintilla/Qt4Qt5/qscicommandset.cpp#L55).
On Linux/Windows, some of these overload existing OpenSCAD application shortcuts (e.g. Ctrl-T which is bound to Insert Viewport Translation). On Mac, some of QScintilla's shortcuts use the Ctrl-key, leaving the Cmd-key menu shortcuts alone, so there are fewer issues (but still some, e.g. our menu shortcut Cmd-D for Comment Selection).

In addition, we've defined some shortcuts in the Edit menu with a wrong *shortcutContext* (```Qt::WidgetWithChildrenShortcut``` instead of the default ```Qt::WindowShortcut```). This causes menu entries like Redo (Ctrl-Shift-P) not to work.