Only hide feedback if Fill in Blank is readonly

[JdenHartog]

Only hide the feedbackTxt if Fill in Blank <input> element has attribute readonly.
This prevents the feedbackTxt from disappearing when one shift-tabs focus on the correct or shown answer in the <input> element and does a keypress.

I also added the readonly attribute to the <input> element if the show answer button is pressed.

[torinfo]

@JdenHartog Also needs some verification, because I don't immediately see the consequences of this change.

[JdenHartog]

@torinfo I put two Xerte project containing only one Gap Fill online:
One without the fix (the current situation): without Fix
And one with the Fix in this pull request changed in file \offline\gapFill.js : with Fix
For both please try the following:

• type "two" in the gap

• hit Check button ("two" will turn green)

• press shift-tab twice to select the word "two"

• press any key and the feedback text will disappear (in the without Fix version ;-)

• reload page

• type "wrong" in the gap

• hit Check button twice (Show Answers button will appear)

• hit Show Answers button ("two" will turn orange)

• press shift-tab three times to select the word "two"

• press any character and the shown answer will change (in the without Fix version ;-)

• also the feedback text will disappear (in the without Fix version ;-)