Add upload functionality directly to package manager grid

Adds an 'Upload Package' option to the 'Download Extras' split button above the Package Management.

Button opens up the file upload dialog with presets
````js
{
  source: 1,
  path: 'core/packages/'
}
````
Starts upload as soon as file is selected. Once file is uploaded, triggers a scanLocal (without the annoying popup confirmation message) to update the grid store and then refreshes the grid.

Tried to limit file selection to .zip files only, but `Ext.ux.UploadDialog.Dialog` doesn't seem to respect it's own `permitted_extensions` config param :confused: