…ence to it (xamarin#617)

* Repro

* Make messaging center callbacks weak references

* Preserve attribute

* Fix test method name

* Watch for collection of actual delegate target instead of wrapper delegate

* Preserve the original platform instance when changing main page

* Better tests for lambda situations

* Update tests, make callback target a weakreference if it's the subscriber

* Ensure old Platform MessagingCenter subs are gone before creating new Platform

* Add pressed and released events to Button

* Update ButtonRenderer.cs

* Apply safely casting to android button renderer

* Use safety casting for Android buttin renderer

* [Windows] Fix modal pages being laid out below soft buttons (#395)

* Add sample HanselForms and TwitterDemo to ControlGallery (#651)

* [Controls] Add Hanselforms sample

* Remove extra twitter sample

* [Controls]Add TwitterDemo sample

* [Controls] Fix build

* Slider should show user-set value on initial load (#378)

* [UWP] Use toolbar foreground color on primary items (#640)

* Avoid duplicating code in OmPlatform (#591)

* [iOS] Entry should not pass a newline to the next responder (#397)

* UITextField should not return so that the next field does not get passed a newline

* Added code sample

* [XamlC] import members on x:Static and factories (#642)

* [Xaml] support short Properties for PropertyCondition (#645)

* Xamlc compile data triggers (#648)

* [Xaml] DataTrigger and PropertyCondition no longer use a ServiceProvider

* [XamlC] avoid generating ServiceProvider for unused ProvideValue

* fix tests

* Fix comment typo

* [UWP] Fix TextBox style for foreground focus color (#618)

* Adding image to use for CellsGalleryImageUrlCellList UI test

* Update ImageCellListPage to use an image we control;
Update CellsGalleryImageUrlCellList test to wait longer than 1s for images
to load if necessary

* fix nre when changing content in datepickerselected (#494)

* Make CellsGalleryImageUrlCellList test finish early if possible

* [iOS] Change keyboard type while keyboard is visible (#443)

* Change keyboard while changing text

* add sample code

* [Android] Fix NavigationPage dispose crash when it parents a MasterDetailPage (#577)

* fix navigation page dispose crash

* changes after review

* [XamlC] detect duplicate x:Name at compile time (#655)

* [XamlC] detect duplicate x:Name at compile time

* invoking methods with the right arguments produces better results

* Make UWP toolbar display rules consistent with other platforms (#638)

* Allow subscriber to be collected if MessagingCenter is the only reference to it (#617)

* Repro

* Make messaging center callbacks weak references

* Preserve attribute

* Fix test method name

* Watch for collection of actual delegate target instead of wrapper delegate

* Preserve the original platform instance when changing main page

* Better tests for lambda situations

* Update tests, make callback target a weakreference if it's the subscriber

* Ensure old Platform MessagingCenter subs are gone before creating new Platform

* [iOS] Prevent multiple ListView cells from being swiped simultaneously (#578)

* disable multiple cell swipe

* add sample code

* refactored

* convert to weakreference

* remove null setting

* change weakreference setting place

* remove if

* revert isopen changes

* add instructions

* [WinRT/UWP] Apply BackgroundColor to Stepper buttons (#581)

* [WinRT/UWP] Apply BackgroundColor to Stepper buttons

* Add explanatory text; use nameof

* Move explanatory text to a label

* Return group instead of internal class (#461)

* [iOS/Android] Move Map camera to correct region on layout change (#548)

* Move to region on layout change

* remove visibility check

* [iOS] Platform specifics for controlling Picker SelectedIndex change behavior (#540)

* picker selected index could change when picker view is dismissed

* use enum

* [iOS] Ignore intermittent failing test on XTC (#666)

* [UITest] Update to UITest 2.0.5 (#665)

* Rebase the current branch onto upstream latest