Backspace on multiple selections affects only one selection

[marceloramires]

The bug could be reproduced in two ways:

1. Block selection
   a) Alt + select a block containing more than one line, and more than one character per line.
   b) Hit backspace

Expected result: all the selected content is deleted
Actual result: only the first line of the selection is deleted

2. Multiple selection

a) Select any portion of text
b) Ctrl + select any other portion of text
c) Hit backspace

Expected result: all the selections' contents are deleted
Actual result: only the first selection is deleted

[marceloramires]

Proposed fix: #765