Use checkboxes to toggle staging

[joewiz]

Great work so far! I've already made my first commit with the plugin, and it worked very smoothly. Coming from other GUI git clients, such as GitHub Desktop, there are a couple of features I would like to see before I begin using this regularly. I'll put them into issues—this being the first:

Currently staging is controlled with two lists. To stage one or more files, select them in the "unstaged" list and click Stage Selected (or select Stage All).

My proposal would be to collapse these two lists into one, and place a checkbox next to each file. Checking a file stages it; unchecking it unstages it. I think this would simplify the UI for users.

Here screenshot to illustrate this UI in GitHub Desktop:

Note that 1.xml and 3.xml are staged, and 2.xml is unstaged. Note also the checkbox above the list of files which indicates some are stages; clicking this checkbox toggles between staging all and none.