Markdown: Add support for default CSS file.

If a file named WE-Markdown.css exists in the user's local AppData, then
it will be used as a default CSS when displaying Markdown (the default
will not be used if the solution contains its own WE-Markdown.css file).

In order to check this feature, place a CSS file such as
[this one](https://git.epsitec.ch/doc/dev/snippets/2) into
`C:\Users\...\AppData\Local\WE-Markdown.css`.

This is equivalent to pull request #1460 but with SPACEs instead of TABs
for the code formatting.