Fix various issues with `SpawnMultiCursor{Up,Down}`

[dmaluka]

Fix various issues with spawning multicursors via SpawnMultiCursorUp and SpawnMultiCursorDown actions (Alt-Shift-Up and Alt-Shift-Down):

• Visual text width (e.g. tabs width) not respected:

Current behavior: Expected behavior:

• When spawning new cursor, current cursor unexpectedly moved too:

Current behavior: Expected behavior:

• LastVisualX not respected:

Current behavior: Expected behavior:

• Softwrapping not respected.
• RemoveMultiCursor action (Alt-p key) removes not the last spawned cursor but the last one before it.
• Cursor selections are not unset when spawning new cursors, which causes weird user experience.