Added Key to GridViewColumnHeader for BasedOn

Added a Key to GridViewColumnHeader so it can be used in based on styles
MetroTabControl uses MetroTabItems instead of regular ones when data binding.
Forgot to override the MetroAnimated*TabControls.
Fixes #411 by setting a font-size on the Clear button.

Signed-off-by: Amrykid <amrykid@gmail.com>
Merge pull request #455 from Amrykid/textbox-fontsize-clearbutton-bug

Fixes #411
Merge pull request #449 from stefan-schweiger/patch-2

Added Key to GridViewColumnHeader for BasedOn
Merge pull request #451 from Amrykid/tabcontrol-item-override

MetroTabControl uses MetroTabItems