Simplified creation of new GUI widgets

[ImpulseAdventure]

Currently, the GUIslice library provides a number of built-in element types (ie. "widgets") in the core library GUIslice.c. A number of extended elements types have been provided in GUIslice_ex.c, including textboxes, sliders, graphs, etc.

In order to increase the ease for users in creating new elements types, I would like to eliminate the GUIslice_ex file and instead provide individual widgets in /src/elem/XWidget.c (for example). This way, it would be a simple matter to create new widgets simply by adding them to the /src/elem folder, and not needing to deal with a monolithic file.

/src/elem/XCheckbox.*
/src/elem/XGauge.*
/src/elem/XSlider.*
...

User sketches could simply include the desired extended widget types at the top of the file, rather than needing to enable special FEATURE #defines.

Another advantage of this approach is that it would be easy for users to submit new widget types to the GUIslice repository by uploading a new standalone file, rather than having to merge with existing GUIslice code.

UPDATE:

• Changed directory from /src/extra to /src/elem
• Added wiki guidance page: https://github.com/ImpulseAdventure/GUIslice/wiki/Note-_-Include-Extended-Elements

[ImpulseAdventure]

Library change has now been integrated.