Patch by Matt Ebb: upgraded usablitiy of text button.

Textbuttons now allow a selection too (like any textbutton in other UIs).
By default, on activating a textbutton, the entire button text is selected
when you enter the button. A single arrowkey or LMB click reveils the
cursor then. Here's more user notes:

LMB click: If inside the button, places the text cursor at the clicked
position. If outside the button, confirms/finishes editing

LMB drag: Selects the text between the start and end point of the drag.

Backspace: Deletes selected text, or backspaces a character

Shift Backspace: Deletes all, as before.

Delete: Deletes selected text or forward deletes a character

Shift LeftArrow: Extends the selection left

Shift RightArrow: Extends the selection right

LeftArrow: If there's a selection, move the cursor to the left edge of the
selection, otherwise move the cursor left a character.

RightArrow: If there's a selection, move the cursor to the right edge of
the selection, otherwise move the cursor right a character.

UpArrow/Home: Move the cursor to the beginning of the line

DownArrow/End: Move the cursor to the end of the line

Ctrl Left arrow and Ctrl Right arrow to jump between directory separators

source/blender/include/interface.h

@@ -63,7 +63,11 @@
 #define PNL_TABBED	8
 #define PNL_OVERLAP	16
 
-
+/* Button text selection:
+ * extension direction, selextend, inside ui_do_but_TEX */
+#define EXTEND_LEFT		1
+#define EXTEND_RIGHT	2
+#define SELWIDTH		(but->selend - but->selsta)
 
 typedef struct {
 	short xim, yim;
@@ -103,7 +107,7 @@ typedef struct {
 
 struct uiBut {
 	uiBut *next, *prev;
-	short type, pointype, bit, bitnr, retval, strwidth, ofs, pos;
+	short type, pointype, bit, bitnr, retval, strwidth, ofs, pos, selsta, selend;
 	int flag;
 	
 	char *str;