Use 'selectmode=mouse' setting for external selects

If Visual Studio programmatically selects some text,
e.g. by double-clicking in the error list, it's true that
the user did not select the text with the mouse, but
they probably want us to assume that they did.
Merge pull request #1280 from ricksladkey/fixes-external-select

Use 'selectmode=mouse' setting for external selects