Fix bad rendering of BBCode tables in RichTextLabel

Text overflowed canvas as tables didn't calculate correctly the width
of their columns. They used the whole table width available for each
column. Also, the `cell` parameter was wrongly parsed if used with its
optional argument (expand ratio).

This PR fixs the parsing of `cell` parameter (i.e. `cell=e`) and the
distribution of the full table width between columns, but it overrides
automatically the `expand` flag if the column is smaller than it could
be, to allow a better UX out-of-the-box. It keeps the `expand_ratio`
flag to let the user customize how every column grows in relation to
the rest.

Partially fix #6289.

scene/gui/rich_text_label.h

@@ -87,6 +87,7 @@ private:
 		int height_accum_cache;
 		int char_count;
 		int minimum_width;
+		int maximum_width;
 
 		Line() {
 			from = NULL;
@@ -199,6 +200,7 @@ private:
 			bool expand;
 			int expand_ratio;
 			int min_width;
+			int max_width;
 			int width;
 		};