Multiline textboxes

[kotek900]

changes:

• holding enter now creates multiple lines (only without RAYGUI_STANDALONE)
• cursor correctly shows up on the x axis (except for an off by one error somewhere)
• cursor is now moves on the y axis
• added GetTextTotalLines (with an ability to limit the amount of characters counted)
• fixed GetTextWidth GetTextWidth() returns incorrect value for multi-line strings #415

todo:

• text wrapping
• turning on multiline mode
• mouse controls
• up/down keyboard controls
• other stuff probably

testing the code:

1. change bool multiline = false; to bool multiline = true;
2. make a GuiTextBox, write some text