Added buttons for a better mobile experience

[johnw42]

This PR adds a series of toggle buttons at the top when running on a mobile platform. The left two buttons simply enter "/" or "#" into the search bar, but the remaining buttons toggle virtual modifier keys, allowing the user to execute a hotkey by typing a letter on the onscreen keyboard.

Here is what the buttons look like upon opening the command palette:

Here is what the input box looks like after the Ctrl and Shift buttons have been tapped: