Add FontSize markup extension

[GeertvanHorrik]

Working on a new idea to allow for "relative" font sizes in any control. The reason is the need to be able to increase the app-wide font size in an application, but it's hard to scale "predefined font size" values (e.g. controls with FontSize 16).

With this markup extension, instead of using

FontSize="16"

we can use

FontSize="{orctheming:FontSize Delta=4}"

(default font size is 12).

Scaling (%):

<Label Content="This is 1.2"
       FontSize="{orctheming:FontSize Scale=1.2}" />

Absolute delta (e.g. +2 font size):

<Label Content="This is +2"
       FontSize="{orctheming:FontSize Delta=2}" />

Example screenshot

[ceo1647]

Solution is flexible but, still I would rather use StaticResource/DynamicResource for these purposes (my personal opinion).
Other looks good, maybe lack of tests