Changed vertex influence visualization for Sculpt and Tweak/magnet.

It was changed the vertex influence visualization for Sculpt and Tweak/magnet as well the code related to it was reorganized and standardized. It was also fixed a bug in sculpt module.

NOTES:
- the Magnet influence in Tweak/Sculpt mode is now shown by using edges highlighting; [Micheus]
- it was added an option to Preferences dialog for Sculpt and Tweak that makes possible to enable/disable the visualization resource; (suggested by Fonte Boa) [Micheus]
- if Work mode is off (unchecked in the View menu) no visual effect is drawn; [Micheus]
- it was changed the way to adjust the Magnet radius in Sculpt mode in order to be consistent with the same operation in Tweak; (suggested by oort) [Micheus]
- fixed a issue in Sculpt module related to the use of shift key for activate smooth functionality while using any other sculpt mode; [Micheus]
- changed the menu description for Sculpt module from Push/Pull to Pull/Push. (suggested by Optigon) [Micheus]