Add UI for the cursorToolOnLoad pref in the Chrome extension + migration logic

[Rob--W]

While preparing for a release, I encountered the "Don't know how to handle cursorToolOnLoad!" error in the console of the options page. After an investigation, I found that the hand tool preference was converted from a boolean to an integer in #7635.

I fixed the UI and added migration logic - see the commit message for more details.

Test:

1. Run gulp chromium

2. Go to chrome://extensions in Chrome, enable Developer mode and load the extension from build/chromium/.

3. Click on Inspect background page of the extension, and run the following code:

   chrome.storage.local.set({enableHandToolOnLoad: true}, function() {location.reload();});

4. Run the following code to confirm that the preference has been migrated:

   chrome.storage.local.get('enableHandToolOnLoad', console.log); // Should be an empty {}.

5. Click on the "Options" link and confirm that the "Cursor tool on load" dropdown is set to "Hand Tool".

Optional (to check that the migration logic does not interfere with a normal functioning of the migrated pref):

6. Change the dropdown to "Text selection tool", close the options page, reload the extension and open the options page.
7. Confirm that the dropdown value is still at "Text selection".
8. Repeat step 6 and 7, but change from "Text selection" to "Hand tool" instead.

[timvandermeij]

I verified this and it works as intended. Thank you for fixing this!