Various cleanups related to button locking

* Rename uiBut.lockstr to disabled_info
* Remove unreachable code
* Replace duplicated check with assert
* Replace overly ambitious check with assert
* Add comments

source/blender/editors/interface/interface_intern.h

@@ -271,7 +271,8 @@ struct uiBut {
 	uiButToolTipFunc tip_func;
 	void *tip_argN;
 
-	const char *lockstr;
+	/* info on why button is disabled, displayed in tooltip */
+	const char *disabled_info;
 
 	BIFIconID icon;
 	char dt; /* drawtype: UI_EMBOSS, UI_EMBOSS_NONE ... etc, copied from the block */