shift+arrow keys should select text

[mvysny]

When editing the line:

• Shift+left/right arrow keys should select text appropriately
• Shift+Home/end should select the text appropriately to the start/end of the line
• Delete/backspace should delete the selected text
• The selection does not need to be propagated to the server-side nor preserved when reconstructing state from the server-side: the selection information doesn't need to leave the browser.