Implemented the 2nd most user requested new feature: the ability for users to pin Calculator on top of other windows.

Description of the changes:

• A Button has been added to TitleBar and styled identical to the other TitleBar buttons. This button only appears in Standard calculator mode. Clicking on this button calls ViewModePreferences API to switch the window into CompactOverlay mode (and back, when exiting from Always-on-Top mode).
• The ViewModel is updated accordingly when the button is pressed. These updates propagate dependency properties and observable variables back to the UI to show/hide and enable/disable certain UI elements (eg. buttons) and keyboard shortcuts.
• The screen size is stored in LocalSettings whenever the user resizes the screen in Always-on-Top mode. This is used for restoring the last used screen size when the user enters Always-on-Top mode.
• AdaptiveTriggers and SizedChanged event handlers are used to implement a responsive UI: certain buttons are hidden when the screen shrinks and the font sizes vary based on the screen size as well.
• The main expression in the Always-on-Top mode combines an ItemsControl and a TextBlock to render the desired mathematical result/expression. This scrollable expression shows the full mathematical expression when edited. When the user presses the "=" button, it shows the final computed result.
• The resulting functionality matches that as specified in the PM spec here.

How changes were validated:

• Manual testing.

Fixes #78