BTextView: Fixup AdoptSystemColors() and HasSystemColors()

Replace private _UsesSystemColors() with public HasSystemColors()
and make sure to consider the tints when checking if system colors
are used or not.

Document HasSytemColors() in the Haiku Book.

Change-Id: I78de4904d5ddb24b98ad27eb93d4e5ccb330d76a
Reviewed-on: https://review.haiku-os.org/c/haiku/+/8897
Haiku-Format: Haiku-format Bot <[email protected]>
Tested-by: Commit checker robot <[email protected]>
Reviewed-by: waddlesplash <[email protected]>

headers/os/interface/TextView.h

@@ -132,6 +132,7 @@ public:
 			void				GetSelection(int32* _start, int32* _end) const;
 
 			void				AdoptSystemColors();
+			bool				HasSystemColors() const;
 
 			void				SetFontAndColor(const BFont* font,
 									uint32 mode = B_FONT_ALL,
@@ -428,8 +429,7 @@ private:
 			float				_TextHeight();
 			BRect				_TextRect();
 
-			float				_UneditableTint();
-			bool				_UsesSystemColors();
+			float				_UneditableTint() const;
 
 private:
 			BPrivate::TextGapBuffer*	fText;