Simplified FloatingWindowContentHost to ContentControl

[xoma-zver]

This PR introduces a change to the base class of FloatingWindowContentHost by simplifying it to ContentControl. The purpose of this modification is to address the flickering issue of the floating window's title, which was described in bug #85.

• What's changed: Changed the base class of FloatingWindowContentHost from its custom implementation to the standard WPF ContentControl.
• Reason for changes: The flickering issue initially fixed by adding code to _HandleNCCalcSize had resurfaced after the code block was removed in PR #345 fix refresh when moving floating windows #347 aimed at resolving bug Title bar refresh not correct on floating window in some situation #345. By simplifying the FloatingWindowContentHost to ContentControl, the flickering is eliminated without introducing new issues.
• Impacts on existing functionality: Preliminary testing shows that this change fixes the flickering issue without negatively affecting other functionalities.

Additional Information:
This change resolves issue #85 and does not reintroduce bug #345.