[jholzer]

Fix #1: FormatString not working with custom texts

• do not put format string into "format-string-like" curly brackets if if allready looks like a format string
• Consolidate Format string handling in base class
• adapt demo application

Fix #2: Control not scaling correctly with HorizontalAlignment set to "Stretch"

• control´s grid columns were set to auto and Horizontal allignment was set to "Center". TextBlocks were set to "Left"-Alignment. This prevents control to scale correctly in "Stretch"-mode. Buttons must be right-allgined for correct scaling
• extend demo app with GridSplitters, so that scaling can be validated

[Dirkster99]

@jholzer I was not able to merge this PR 50 successfully - Please review PR #51 to see why I had to revert. Thank you.