These actions reintroduce the behavior of micro where the Home key
toggles between the start of text (first) and the start of the line.
The same applies for the variant with selection. This commit also
sets these bindings as the defaults.
The default keybindings now use StartOfText which moves the cursor
to the start of the text on the current line instead of the actual
start of the line (if the line begins with whitespace).
Fixes#1468
On non-Mac operating systems, the default for CtrlLeft/CtrlRight
is now WordLeft and WordRight instead of moving the cursor to the
start and end of lines (now rebound to Alt-Left/Right by default).
Default keybindings are unchanged on Mac.
Fixes#1465
