Microsoft.UI.Xaml v2.1.190219001-prerelease

https://www.nuget.org/packages/Microsoft.UI.Xaml/2.1.190218001-prerelease

New features:

• TeachingTip (87ce7c02 by StephenLPeters, pr #32)

Notable bug fixes:

• Updated NavigationViewItemHeader and PaneTitle to use semibold text (c3ad2f96 by ojhad, pr #316)
• Enable ThemeShadow and XamlRoot codepaths in MUX (1e63a991 by jevansaks, pr #262)
• Add source indexing back to PR/CI builds (50aec202 by jevansaks, pr #292)
• Update build pipelines to generate framework package (c4987e51 by jevansaks, pr #300)
• Fix NavigationViewPane shadow (84748f5f by kaiguo, pr #286)
• check for Control.CornerRadius before loading rs5 generic.xaml (3e433288 by kmahone, pr #297)
• Fix issue with multiple content presenters. (80c52379 by StephenLPeters, pr #289)
• Removing usage of ScrollBar2 (49c3e7e0 by RBrid, pr #265)
• Fix issue with CI build sometimes conflicting on nuget version (f9a707f0 by jevansaks, pr #291)
• TeachingTip test pattern update. (ae2c38d0 by StephenLPeters, pr #283)
• make navviewitem clickable after navview is freely moved between visualtrees. (92b62928 by licanhua, pr #249)
• Backport CommandBarFlyout shadow fix (a348e0b0 by kaiguo, pr #281)
• Avoid name conflicts in mux and wux for some keyed styles (f8ba2f2c by licanhua, pr #271)
• make Narrator announces count for Settings item be 1 of 1 (1ed0b4fe by licanhua, pr #278)
• Update TextCommandBarFlyout to handle selection of mixed format text (7612214b by kmahone, pr #261)
• Selecting custom color on non-100% DPI crashes settings (24b3dcb0 by llongley, pr #247)