Add cTab_fnc_update_txt_size to precalculate various text and icon sizes and call it once at the start, add cTab_fnc_txt_size_inc and cTab_fnc_txt_size_dec to manage increase and decrease of text and icon sizes
Use precalculated text and icon sizes, this should increase performance as calculations are only done when there is a change instead of multiple times every time an icon is drawn on the map. See #48.