Support cmd-a, c, v, x for text input in native dialogs like file open, save

Addresses this bug:

1. File -> Open.
2. Type in some text in the file open dialog (in the search field in Yosemite or filename text input)

Actual: Try Cmd-A, C, X, V - they do not work.
Expected: They work.
It's ok to have Cmd-Y for redo, but most mac apps provide Cmd-Shift-Z as redo. So we also need to handle that.