Catching Line Changes (Combining Lines)

There are two key commands that can cause a line to be deleted (but there is currently no key command to delete a line).  One is deleting when the cursor is at the end of the line, where the current line will be combined with the next, the next line will be deleted.  The other is backspacing when the cursor is at the beginning of the line, where the current line will be deleted, but the contents of the line will be combined with the previous line.

For the delete check, where the cursor is at the end of the line, it can't be at the end of the file (no line will be deleted) and there can't be any text selected (selections will be handled separately).  For the backspace check, where the cursor is at the beginning of the line, it can't be at the beginning of the file (nothing to backspace to) and there can't be any text selected.

Both of these will emit the lines deleted signal for one line, delete will send the current line number plus one and backspace will send the current line number.  Both of these signals are sent before the actual deletion takes place in the edit box.  When the document changed signal is received, the current modified line number will be set (when the cursor leave the line, it will be reported as changed).

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