Additional modal presentation styles

[svara]

This PR adds two modal styles: page sheet and form sheet. These styles differ in presentation only on iPads. On iPhones, they fallback to the same behavior:
• In portrait mode, the system displays the view controller as a sheet, with part of the background content visible near the top of the screen.
• In landscape mode, the behavior is identical to full screen presentation.

Page sheet	Form sheet

Additionally, this PR also adds the option to disable the pull-down gesture for the page and form sheet presentations. This is particularly important on iPads, where all it takes is an accidental tap outside of the sheet to dismiss it.
You can specify the behaviour by adding modal_sheet_dismiss_gesture_enabled to the path config. If omitted, the default value true is used, which matches the default OS behaviour.

{
   "rules":[
      {
         "patterns":[
            "/recipes/*"
         ],
         "properties":{
            "context": "modal",
            "modal_sheet_dismiss_gesture_enabled": false,
            "view_controller":"recipes"
         }
      }
   ]
}

[jayohms]

Awesome, thanks!

[joemasilotti]

I hate to come in late after a merge, but I think these should be:

case pageSheet = "page_sheet"
case formSheet = "form_sheet"

to match Navigation.Presentation.clearAll and Navigation.Presentation.replaceRoot.

[joemasilotti]

Addressed in PR #63.