Various insert keyboard shortcut improvements

[michielp1807]

Closes #6, and implements some other features as well:

• Ctrl+Backspace and Ctrl+Delete to remove entire words at once (similar to Ctrl+Left and Ctrl+Right navigation)
• Alt+Up and Alt+Down to move the current line up or down
• Home and End to move to the beginning or end of the current line
• Ctrl+Home and Ctrl+End to move to the beginning or end of the file
• Page Up and Page Down to quickly navigate up or down
• Ctrl+Page Up and Ctrl+Page Down to quickly navigate up or down while keeping the cursor on the same row (similar to Ctrl+Up and Ctrl+Down navigation)