Don't propagate the mousdown event when any flyout is closed

Fixes #1411
Support runtime accents
Merge pull request #1428 from MahApps/fix-1411

Don't propagate the MouseDown event when any Flyout is closed
Merge pull request #1430 from GeertvanHorrik/master

Support runtime accents
Added templating for the mouse over when the `DataGrid` has `SelectionUnit=Cell`.
Fixes #975
Modified trigger for the cell color when selected

Corrected the issue that the active selection was not the correct color.
Unfortunately, the active selection mechanism doesn't work when the datagrid is in "selection unit cell". Therefore, I used the AccentColor for the Cell selection.
Added DataGrid example for selection unit cell
Merge pull request #1446 from Gimly/master

Fixed templating issues when the DataGrid has SelectionUnit=Cell
simplify datagrid example
set background for DataGridRow
Merge pull request #1448 from MahApps/punker76-simplify-datagrid-example

simplify datagrid example
fix treeview style to allow custom styling
fix some styling colors
Merge pull request #1450 from MahApps/1444-punker76-treeview-styling

fix treeview style to allow custom styling
fix selection with SelectionUnit=CellOrRowHeader
Merge pull request #1452 from MahApps/punker76-DataGrid-CellOrRowHeader

fix selection with SelectionUnit=CellOrRowHeader