Reset grid content when running reset

In the current scrollback PR the `reset` command does not affect the
scrollback history. To make sure the terminal is properly reset, it
should clear the scrollback history.

This commit fixes this by creating a new and empty grid whenever `reset`
is executed. It takes the current dimensions and history size from the
old grid.

Right now there's an empty ref-test called `grid_reset` without any
content, this should be implemented once #1244 is resolved.

This fixes #1242.

tests/ref/grid_reset/size.json

@@ -0,0 +1 @@
+{"width":1259.0,"height":1380.0,"cell_width":10.0,"cell_height":19.0,"padding_x":5.0,"padding_y":5.0}

tests/ref/history/alacritty.recording

@@ -1,4 +1,4 @@
-%                                                                               
 

 UL  ~/…/tests/ref/history  issue-1244  [?2004h

 UL  ~/…/tests/ref/history  issue-1244  for i in {50000..2}; do echo $i; donefor i in {50000..2}; do echo $i; done[?2004l
+%                                                                                                                           
 

 UL  ~/…/tests/ref/history  issue-1244  [?2004hfor i in {50000..2}; do echo $i; donefor i in {50000..2}; do echo $i; done[?2004l
 50000
 49999
 49998
@@ -49998,4 +49998,4 @@
 4
 3
 2
-%                                                                                                                                                                                                                                                           
 

 UL  ~/…/tests/ref/history  issue-1244  [?2004h
+%                                                                                                                           
 

 UL  ~/…/tests/ref/history  issue-1244  [?2004h

tests/ref/history/size.json

@@ -1 +1 @@
-{"width":2532.0,"height":1380.0,"cell_width":10.0,"cell_height":19.0,"padding_x":5.0,"padding_y":5.0}
+{"width":1259.0,"height":1380.0,"cell_width":10.0,"cell_height":19.0,"padding_x":5.0,"padding_y":5.0}