Slider widget for options window (#215)

* Add draft of new slider widget type.

* Add new OPT_FLOAT_S and OPT_INT_S option types to elconfig, utilising the new slider widget.

* Enable the right-click context menu for the new slider option types to allow direct defaulting/resetting.

* Update several candidate options to use the new slider option types.

* Tidy up set_var functions a little.

* Add a "current value" line to the context menu for slider options.

* Small fix for option context menu checkbox positioning when current value is displayed.

* Fix improper code indentation.

* Fix small compile warning in widgets.c.

* Limit context menu entries for slider options to 2 significant figures.

* Removed the erroneous SDL_StartTextInput for Android in slider_onclick_handler.

* Fix integer truncation bug causing the slider widgets to not function correctly at different UI scaling levels.

* Resolve some Android-related issues with the new slider widget.

* Slider patch for Android compatibility.

* Check add_cm_current_value_line executes correctly before making the resultant context menu line grey.

* Add a click sound for slider onclick events.

Author: bendoughty