Fix warning when building with Qt 4

Change-Id: I52b6b84732cf363b06ea248eeabdc99d89fb4beb
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Turn struct ProjectExplorerSettings into a class

We agreed to not use structs to avoid the class/struct mismatch
warnings in MSVC and clang.

Change-Id: Id1b2d89646d3a982c8d85b84b44a4e697102c220
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>