Implement date-time pickers

[classilla]

https://blog.nightly.mozilla.org/2017/06/12/datetime-inputs-enabled-on-nightly/

The worklist is mostly here: https://wiki.mozilla.org/TPE_DOM/Date_time_input_types

Originally this started as https://bugzilla.mozilla.org/show_bug.cgi?id=825294 and landed as https://hg.mozilla.org/mozilla-central/rev/14ac8b409bcd in Fx50.

The metabug is https://bugzilla.mozilla.org/show_bug.cgi?id=888320 .

[classilla]

Then, implement https://bugzilla.mozilla.org/show_bug.cgi?id=1368888

[classilla]

https://developer.apple.com/documentation/appkit/nsdatepicker?language=objc

[classilla]

Current plan:

• Create as a regular text box.
• On click, programmatically open a date panel (like ComplexTextInputPanel.mm with an NSDatePicker
• Fill the box with YYYY-MM-DD

This should be enough to emulate the current behaviour.

[classilla]

diff --git a/dom/html/HTMLInputElement.cpp b/dom/html/HTMLInputElement.cpp
index 9e544bd3b..15434e08a 100644
--- a/dom/html/HTMLInputElement.cpp
+++ b/dom/html/HTMLInputElement.cpp
@@ -2958,6 +2958,12 @@ HTMLInputElement::Select()
     return NS_OK;
   }
 
+#if(1)
+  if (mType == NS_FORM_INPUT_DATE) {
+    fprintf(stderr, "Date picker selected, call SetUserInput when ready\n");
+  }
+#endif
+
   if (!IsSingleLineTextControl(false)) {
     return NS_OK;
   }

[classilla]

The pickers are initialized in HTMLInputElement::MaybeInitPickers.

[classilla]

This can be closed in lieu of the followup issues.