[lib] working variant->string->variant

QVariant ctor constructs string, also QVariant.toString() gives
"true|false" as output. Contextkit subscribers expects types
corresponding to source types. It is almost impossible to implement
correct type passing w/o breaking the rule of trying to expose string in
statefs file for almost all structured types, so just adding quick,
dirty and slow (using sequential regexp matching performance is
non-optimal) but working conversion. Possible optimizations: any normal
parser or xattr for statefs (if file content type is not changing in
time)

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[cmake] pic/pie flags in proper makefiles

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[subscriber] check notifier before using

in some scenarios it can be not initialized yet

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
timer is owned by ContextPropertyPrivate

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[subscriber] using separate thread for polling

Properties are monitored in a separate thread because during animation
main thread polls 'em frequently

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>