Add link to TextMate Theme preference page in Syntax Coloring preference page

[ghentschke]

User are confused when they want to change the Syntax Coloring of the editor. Since the LSP based C/C++ editor relies on TM4E themes we should add a link to the TextMate -> Theme preference page in a new Editor (LSP) Syntax Coloring sub-page.

[ghentschke]

See also #455 and #456