BTextView: Use document colors on AdoptSystemColors()

AdoptSystemColors() tints document background color to match panel
color if uneditable.

Calling MakeEditable() will automatically apply or unapply uneditable
background tint if you have previously called AdoptSystemColors().

Parent BView::AdoptSystemColors() sets panel colors, we want document
colors here. Do not alter text color - only view, low and high colors
are changed.

Document AdoptSystemColors() and MakeEditable() in BTextView docs.

Change-Id: Ib215735f27bb01fc2f95fcf2fee0185e5fc83f70
Reviewed-on: https://review.haiku-os.org/c/haiku/+/8263
Haiku-Format: Haiku-format Bot <[email protected]>
Reviewed-by: waddlesplash <[email protected]>
Tested-by: Commit checker robot <[email protected]>

headers/os/interface/TextView.h

@@ -131,6 +131,8 @@ public:
 			void				SelectAll();
 			void				GetSelection(int32* _start, int32* _end) const;
 
+			void				AdoptSystemColors();
+
 			void				SetFontAndColor(const BFont* font,
 									uint32 mode = B_FONT_ALL,
 									const rgb_color* color = NULL);
@@ -426,6 +428,9 @@ private:
 			float				_TextHeight();
 			BRect				_TextRect();
 
+			float				_UneditableTint();
+			bool				_UsesSystemColors();
+
 private:
 			BPrivate::TextGapBuffer*	fText;
 			LineBuffer*			fLines;