add attached property UseSpellCheckContextMenu
Fix #1051
Fix #1052
finished changes

- rename property to IsSpellCheckContextMenuEnabled
- don't show separator if suggestions are empty
- raise InvalidOperationException if element isn't a TextBoxBase
- add property to TextBox and RichTextBox at main window example
use opacity mask to avoid warnings
Merge pull request #1061 from MahApps/955-punker76-flyout-binding-warnings

fix flyout warnings
Merge pull request #1056 from MahApps/1048-punker76-textbox-spellchecking-menu

enable spell checking context menu
Merge pull request #1057 from xxMUROxx/hotfix/NumericUpDown

Hotfix/numeric up down
increase scrollbar repeat buttons and thumb
fix the redraw bug

set only AllowsTransparency="true" and all works fine
Merge pull request #1065 from MahApps/985-punker76-combobox-non-aero-redraw-fix

fix for combox popup redrawn bug
Merge pull request #1064 from MahApps/1033-punker76-scrollbar-size

increase scrollbar repeat buttons and thumb
add validation example to flyouts demo (for #968)
don't set to transparent on editable
Merge pull request #1069 from MahApps/1037-punker76-editable-combobox-background

fix for editable combobox background
Update XAML to make lower thumb really lower and Upper really upper.
- Made corresponded changes to correctly handle new behavior
- Fix MoveToPoint issues
- Code refactoring and optimizations
Update Metro.Demo for range Slider
Merge pull request #1072 from QuantumDeveloper/RangeSlider

Range slider V2.2
Can set Handled for Increment/Decrement/ValueChanged

Closes #1075