Align Borders on Controls

[wolframhaussig]

This is the followup issue on: #13 (comment)

I've taken the liberty to update our example project with the current/new default values introduced in this project.
In the screenshot below you can see a noticable difference between StylableDateTimePicker (no border), StylableComboBox (kind of 3D) and StylableButton (full border):

I'm not sure about how we can approach this the best, though. The "full border solution" on the buttons were intentional in the beginning since we need this for our dark designs, the other variants are debatable.

[wolframhaussig]

@Nockiro we should discuss this further. Maybe it makes sense to create a reusable utility class handling the different border styles (similar to this solution for FormBorderStyle).