fix invalid cursor states and visual selection issues

[mheister]

I found that pressing dd on the last line twice would make the program crash and made changes to prevent the cursor staying in invalid positions after dd and deletion of a visual selection. As I found it hard to test these things separately, this also changes the behavior of visual selections such that selections with starting line below end line can be yanked/deleted and prevents the internal end position of the selection to go negative.

[spytheman]

@medvednikov what do you think?