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'.
[tests] Import ut_mthemedaemonprotocol from libmeegotouch

Imported from revision fc20afc251f00bb7637da48c57eccb199a842367
ut_mthemedaemonprotocol: include in build again
[tests] Import ut_mcommonpixmaps from libmeegotouch

Imported from revision fc20afc251f00bb7637da48c57eccb199a842367
ut_mcommonpixmaps: include in build again
[tests] Import ut_mimagedirectory from libmeegotouch

Imported from revision fc20afc251f00bb7637da48c57eccb199a842367
ut_mimagedirectory: include in build again
Tests: always use exec wrapper

Prerequisite for next commit.
Extract testutils from application code

Certain classes, formely part of libmeegotouch/corelib/theme, are now
used by test code only - extracted into a library shipped with testutils
subpackage.

Note that INotify global instance is no more stored by value - this
fixes segfault on exit.