Fix snippet compiler sample.

Remaining problems:

Application can hang after debugging.
Selecting item in Errors list does not jump to text editor line.
Fix debugger hang in snippet compiler sample.
Make snippet compiler main window bigger.
Version increment
Fix "ArgumentException: An item with the same key has already been added" when an attribute has multiple named arguments with the same name.
Fix standard headers not being saved.
Add MVC 4 project templates.
Rename MVC 3 project templates.
Update installer with new MVC project templates.
Clear property container on designer unload so that the ComboBox of selectable objects stays empty when the designer cannot be reloaded due to syntax errors.
Make AvalonEdit xshd schema more flexible: allow elements and attributes from other namespaces.
Fix Get-Package output in NuGet console window.
Use processContent="lax" as recommended in http://xml.coverpages.org/HP-StephensonSchemaBestPractices.pdf
Fix compiler warnings.
add highlighting definition for MarkDown syntax
Check known file extensions for icons when collecting usage data.

Check the file extension against file filters first and then
check the file extensions that have an associated icon.
Add .aspx file filter.
Fix several issues with SearchPanel.
Allow running actions as part of file templates.
further improvements and bugfixes for MarkDown-Mode