Add vim's 'ctrl+d','ctrl+u','g g', and 'G' actions

* 'd' - scroll down half page
* 'u' - scroll up half page
* 'g g' - scroll to top of page
* 'G' - scroll to bottom of page
Make escape blur inputs and clear hints

This effectively makes a 'normal mode' feeling by allowing you to
always get out of the other modes. Especially effective if you don't use
a modifier key to prefix commands.
Fake an 'insert mode' by pressing 'i'

Really, it is just a user-requested way to stop actions from happening.
Allow binding multiple keys to an action

Keycode preferences can now be a comma-separated list of hotkeys, which
allows you to set multiple hotkeys to the same action.

This is useful if more than one hotkey makes sense, for example
'ctrl+d' and 'd', and you can't stop switching between them.
Stop Vimari from capturing capital letters in text fields

This fixes a bug where Vimari would capture key combinations
involving only the SHIFT modifier in text areas. This behavior
was added in commit:c2b3e0386297 and causes problems when
typing capital letters if you have any Vimari commands configured
which are only `shift+_`.
Open up link hint characters for user modification