Various TextObject and TextArray additions and fixes

[Vankata453]

Fixes various issues, regarding TextObject and TextArray:

• Fixes #2256: Adds the missing set_front_fill_color(), set_back_fill_color(), set_text_color() and set_roundness() functions to TextArray, which only modify the current TextObject.
• Closes #2257: The set_text() function of TextArray now adds a new TextObject with default duration set, if the TextArray has no current TextObject.
• Closes #2258: Toggling the visibility of a TextObject now checks if the desired visibility state is true or false. If true, fade_in(0) is executed, if false - fade_out(0). This would allow not just the frame to be toggled, but the text as well.