Make it easier to delete the last characters of a song line.

When using song text, it is often appropriate to correct the last characters typed, e.g. if a typo occured. Therefore, it should be possible to jump to the last text entry by pressing the "Delete" key after one has input an empty space or a minus sign.

Currently, one stays at the begin of the current text entry when pressing the "Delete" key after an empty space character and has to manually change the text entry cell..