Fix warnings about unused variables
[tests] Do not build custom daemon for stresstest
Tests: cleanup references to application code
Cleanup project files

Notes:

1. Ut_MRemoteThemeDaemon will not be imported from libmeegotouch as
   MRemoteThemeDaemon is not part of mthemedaemon API and is only used
   in test code - removing 'friend class' declaration.
2. IMAGESDIR was only used on single place while there are file paths
   hardcoded on many other places - removing now, maybe replacing
   globally in future.
Extract common qmake stuff into common.pri

Note: enabling QT_USE_FAST_OPERATOR_PLUS required minor adjustments to
QDir initialization.
Use temporary static lib to share code with tests
Move tests under tests/ directory

This also renames the 'mthemedaemontest' binary to 'stresstest'.