Examples: Hard-code the 'main' file in .qmlproject

Examples: Hard-code the 'main' file in .qmlproject

Set the 'mainFile:' property in .qmlproject files where it makes sense (i.e.
there's one natural candidate). This makes sure Qt Creator doesn't fall back
to 'current file in editor' as main file, which is unexpected to a lot of people.

(This requires Qt Creator version 2.2 and newer).

Change-Id: Ia5d20b0d982038520d7892bca155267c5f13644b
Reviewed-by: Martin Jones <martin.jones@nokia.com>