Improve Starting Line Number dialog

[ProgerXP]

• Assign Ctrl+Alt+Shift+N hotkey (unless it's used) to this command.
• Add Syslinks as in Alt+Q: 0 1 (set the input's value to this number and close dialog), -1 +1 (adjust the value by delta; both with tooltips: Arrow Down Arrow Up)
• Handle Arrow Up and Down keypresses while the input is focused that increment/decrement the value by 1

#342

[cshnik]

Done.