Refactor of ModernUI_Button

[mrfearless]

Todo:

• Remove some properties relating to notify icon and features, think this can be handled in another manner external to the ModernUI_Button (perhaps? - feedback to see if its useful or just adding bloat to the control)
• Use the pre calculation function as was intended to speed up drawing of the button (or remove altogether and continue using existing way) - feedback required to see if its worth going down this route.
• Possibly add gradient coloring to the background - but would require at least 4/5 additional properties to handle the 'to' gradient color, existing properties for backcolor can be used for the 'from' part of the gradient. Some logic like if @ButtonBackColorGradTo != 0 then use @ButtonBackColor as 'from' gradient color and @ButtonBackColorGradTo as 'to' gradient color.
• Possibly add in rounded rect for borders, without using GDI+. Issues that might prevent this is corners outside of roundrect showing back color, or clipping showing black - this is always an issue, hence why it hasnt been implemented - SetRegion isnt anti-aliased so it can be noticable. Also the more extreme the rounding the more obvious via GDI the blockiness of corners. Lower values 2,3,4,5 for roundrect appear the best looking, but still not great.
• Center image and bottom text still not tested - suspect it needs a good review and changes to properly have it working.